There are several key words in this definition:
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Use-case instance. The sequence referred to in the definition is really a specific flow
of events through the system, or an instance. Many flows of events are possible, and many may be very
similar. To make a use-case model understandable, you should group similar flows of events into one use
case. Identifying and describing a use case really means identifying and describing a group of related
flows of events.
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System performs. This means that the system provides the use case. An actor communicates
with a use-case instance of the system.
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An observable result of value. You can put a value on a successfully performed use case. A use
case should make sure that an actor can perform a task that has an identifiable value. This is very
important in determining the correct level or granularity for a use case. Correct level refers to
achieving use cases that are not too small. In certain circumstances, you can use a use case as a
planning unit in an organization that includes individuals who are actors in the system.
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Actions. An action is a computational or algorithmic procedure. It is invoked either when the
actor provides a signal to the system or when the system gets a time event. An action may imply signal
transmissions to either the invoking actor or other actors. An action is atomic, which means it is
performed either entirely or not at all.
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A particular actor. The actor is key to finding the correct use case, especially because
the actor helps you avoid use cases that are too large. As an example, consider a visual modeling tool.
There are really two actors to this application: a developer - someone who develops systems using the
tool as support; and a system administrator - someone who manages the tool. Each of these actors has
his own demands on the system, and will therefore require his own set of use cases.
The functionality of a system is defined by different use cases, each of which represents a specific flow
of events. The description of a use case defines what happens in the system when the use case is performed.
In an automated teller machine the client can, for instance, withdraw money from an account, transfer money
to an account, or check the balance of an account. These functions correspond to flows that you can
represent with use cases.
Each use case has a task of its own to perform. The collected use cases constitute all the possible ways of
using the system. You can get an idea of a use-case task simply by observing its name.
Following is a set of questions that are useful when identifying use cases:
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For each actor you have identified, what are the tasks in which the system would be involved?
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Does the actor need to be informed about certain occurrences in the system?
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Will the actor need to inform the system about sudden, external changes?
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Does the system supply the business with the correct behavior?
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Can all features be performed by the use cases you have identified?
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What use cases will support and maintain the system?
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What information must be modified or created in the system?
Use cases that are often overlooked, since they do not represent what typically are the primary functions
of the system, can be of the following kind:
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System start and stop.
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Maintenance of the system. For example, adding new users and setting up user profiles.
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Maintenance of data stored in the system. For example, the system is constructed to work in parallel
with a legacy system, and data needs to be synchronized between the two.
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Functionality needed to modify behavior in the system. An example would be functionality for creating
new reports.
If you have developed a business use-case model and a business analysis model, these artifacts are a good starting
point for identifying use cases.
In early iterations in elaboration, only a few use cases (those that are considered architecturally
significant) are described in any detail beyond the brief description. You should always first develop an
outline of the use case (in step-by-step format) before delving into the details. This step-by-step outline
should be your first attempt at defining the structure of the flow of events of the use case (see Flow of Events - Structure below). Always start with the basic flow
of the use case. Once there is some agreement on the outline of the basic flow, you can add what the
alternative flows should be in relation to the basic flow.
Towards the end of elaboration, all use cases you plan to describe in detail should be completed.
There will often be use cases in your model that are so simple that they do not need a detailed description
of the flow of events, a step-by-step outline is quite enough. The criteria for making this decision is
that you don't see disagreement among user kind of readers on what the use case means, and that designers
and testers are comfortable with the level of detail provided by the step-by-step format. Examples are use
cases that describe simple entry or retrieval of some data from the system.
It is often hard to decide if a set of user-system interactions, or dialog, is one or several use cases.
Consider the use of a recycling machine. The customer inserts deposit items, such as cans, bottles, and
crates, into the recycling machine. When she has inserted all her deposit items, she presses a button, and
a receipt is printed. She can then exchange this receipt for money.
Is it one use case to insert a deposit item, and another use case to require the receipt? Or is it all one
use case? There are two actions, but one without the other is of little value to the customer. Rather, it
is the complete dialog with all the insertions, and getting the receipt, that is of value for the customer
(and makes sense to her). Thus, the complete dialog, from inserting the first deposit item, to pressing the
button and getting the receipt, is a complete case of use, a use case.
Additionally, you want to keep the two actions together, to be able to review them at the same time, modify
them together, test them together, write manuals for them and in general manage them as a unit. This
becomes very obvious in larger systems.
A use case describes what happens in the system when an actor interacts with the system to execute the use
case. The use case does not define how the system internally performs its tasks in terms of collaborating
objects. This is left for the use-case realizations to show.
Example:
In the telephone example, the use case would indicate - among other things - that the system issues a
signal when the receiver is lifted and that the system then receives digits, finds the receiving party,
rings his telephone, connects the call, transmits speech, and so on.
In an executing system, an instance of a use case does not correspond to any particular object in the
implementation model (for example, an instance of a class in the code). Instead, it corresponds to a
specific flow of events that is invoked by an actor and executed as a sequence of events among a set of
objects. In other words, instances of use cases correspond to communicating instances of implemented
objects. We call this the realization of the use case. Often, the same objects participate in realizations
of more than one use case. For example, both the use cases Deposit and Withdrawal in a banking system may
use a certain account object in their realization. This does not mean that the two use cases communicate,
only that they use the same object in their realization.
You can view a flow of events as consisting of several subflows, which taken together yield the total flow
of events. You can reuse the description of a subflow in other use cases' flow of events. Subflows in the
description of one use case's flow of events may be common to those of other use cases. In the design you
should have the same objects perform this common behavior for all the relevant use cases; that is, only one
set of objects should perform this behavior, no matter which use case is executing.
Example:
In an automated teller machine system the initial subflow is the same in the flow of events of the use
cases Withdraw Money and Check Balance. The flow of events of both use cases start by checking the identity
of the card and the client's personal access code.
A use-case instance can follow an almost unlimited, but enumerable, number of paths. These paths represent
the choices open to the use-case instance in the description of its flow of events. The path chosen depends
on events. Types of events include:
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Input from an actor. For example, an actor can decide, from several options, what to do next.
Example:
In the use case Recycle Items in the Recycling-Machine System the Customer always has two options: hand in
still another deposit item or get the receipt of returned items.
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A check of values or types of an internal object or attribute. For example, the flow of events may
differ if a value is greater or less than a certain value.
Example:
In the use case Withdraw Money in an automated teller machine system, the flow of events will differ if the
Client asks for more money than he has in his account. Thus, the use-case instance will follow different
paths.
Instances of several use cases and several instances of the same use case work concurrently if the system
permits it. In use-case modeling, you can assume that instances of use cases can be active concurrently
without conflict. The design model is expected to solve this problem, because use-case modeling does not
describe how things work. One way to view this is to assume that only one use-case instance is active at a
time and that executing this instance is an atomic action. In use-case modeling, the "interpreting machine"
is considered infinitely fast, so that serialization of use case instances is not a problem.
Each use case should have a name that indicates what is achieved by its interaction with the actor(s). The
name may have to be several words to be understood. No two use cases can have the same name.
Example:
These are examples of variations of the name for the use case Recycle Items in the Recycling Machine
example:
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Receive Deposit Items
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Receiving Deposit Items
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Return Deposit Items
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Deposit Items
The brief description of the use case should reflect its purpose. As you write the description, refer to
the actors involved in the use case, the glossary and, if you need to, define new concepts.
Example:
Following are sample brief descriptions of the use cases Recycle Items and Add New Bottle Type in the
Recycling-Machine System:
Recycle Items: The user uses this machine to automatically have all the return items (bottles, cans,
and crates) counted, and receives a receipt. The receipt is to be cashed at a cash register (machine).
Add New Bottle Type: New kinds of bottles can be added to the machine by starting it in 'learning
mode' and inserting 5 samples just like when returning items. In this way, the machine can measure the
bottles and learn to identify them. The manager specifies the refund value for the new bottle type.
The Flow of Events of a use case contains the most important information derived from use-case
modeling work. It should describe the use case's flow of events clearly enough for an outsider to easily
understand it. Remember the flow of events should present what the system does, not how the system is
design to perform the required behavior.
Guidelines for the contents of the flow of events are:
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Describe how the use case starts and ends.
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Describe what data is exchanged between the actor and the use case.
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Do not describe the details of the user interface, unless it is necessary to understand the behavior of
the system. For example, it is often good to use a limited set of web-specific terminology when it is
known beforehand that the application is going to be web-based. Otherwise, your run the risk that the
use-case text is being perceived as too abstract. Words to include in your terminology could be
"navigate", "browse", "hyperlink" "page", "submit", and "browser". However, it is not advisable to
include references to "frames" or "web pages" in such a way that you are making assumptions about the
boundaries between them - this is a critical design decision.
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Describe the flow of events, not only the functionality. To enforce this, start every action with "When
the actor ... ".
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Describe only the events that belong to the use case, and not what happens in other use cases or
outside of the system.
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Avoid vague terminology.
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Detail the flow of events-all "whats" should be answered. Remember that test designers are to use this
text to identify test cases.
If you have used certain terms in other use cases, be sure to use the exact same terms in this use case,
and that their intended meaning is the same. To manage common terms, put them in a glossary.
The two main parts of the flow of events are basic flow of events and alternative
flows of events. The basic flow of events should cover what "normally" happens when the use case is
performed. The alternative flows of events cover behavior of optional or exceptional character in relation
to the normal behavior, and also variations of the normal behavior. You can think of the alternative flows
of events as "detours" from the basic flow of events, some of which will return to the basic flow of events
and some of which will end the execution of the use case.
The typical structure of the flow of events. The straight arrow represents the basic flow of events, and
the curves represent alternative paths in relation to the normal. Some alternative paths return to the
basic flow of events; whereas others end the use case.
Both the basic flow of events and the alternative flows events should be further structured into steps or
subflows. In doing this, your main goal should be readability of the text (see also the section Flow of Events - Style below). A rule of thumb is that a subflow should
be a segment of behavior within the use case that has a clear purpose, and is "atomic" in the sense that
you do either all or none of the actions described. You may need to have several levels of subflows, but if
you can you should avoid it since it makes the text more complex and harder to understand. You can
illustrate the structure of the flow of events with an activity diagram, see Work Product Guideline: Activity Diagram in the Use Case.
This type of written text, structured into consecutive subsections, will by its nature imply to the reader
that there is a sequence between the subflows. To avoid misunderstandings, you should always point out
whether the order of the subflows is fixed or not. Considerations of this kind are often related to:
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Business rules. For example, the user has to be authorized before the system can make certain data
available.
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User-interface design. For example, the system should not enforce a certain sequence of behavior that
may be intuitive to some but not to other users.
To clarify where an alternative flow of events fits in the structure, you need to describe the following
for each "detour" to the basic flow of events:
-
Where in the basic flow of events the alternative behavior can be inserted.
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The condition that needs to be fulfilled for the alternative behavior to start.
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How and where the basic flow of events is resumed, or how the use case ends.
Example:
This is an alternative subflow in the use case Return Items in the Recycling-Machine System.
2.1. Bottle Stuck
If in section 1.5, Insert Deposit Items, a bottle gets stuck in the gate, the sensors around the gate and
the measuring gate will detect this problem. The conveyer belt is stopped and the machine issues an alarm
to call for the operator. The machine will wait for the operator to indicate that the problem has been
fixed. The machine then continues in section 1.9 of the basic flow.
In the example above, the alternative flow of events is inserted at a specific location in the basic flow
of events. There are also alternative flow of events that can be inserted at more than one location, some
can even be inserted at any location in the basic flow of events.
Example:
This is an alternative subflow in the use case Return Items in the Recycling-Machine System.
2.2. Front Panel is Removed
If somebody removes the front panel to the Recycling machine, the can compression is deactivated. It will
not be possible to start the can compression with the front panel off. The removal will also activate an
alarm to the operator. When the front panel is closed again, the machine resumes operation from the
location in the basic flow of events at which it was stopped.
It might be tempting, if the alternative flow of events is very simple, to just describe it in the basic
flow of events section (using some informal "if-then-else" construct). This should be avoided. Too many
alternatives will make the normal behavior difficult to see. Also, including alternative paths in the basic
flow of events section will make the text more "pseudo-code like" and harder to read.
In general, extracting parts of the flow of events and describing these parts separately, can increase the
readability of the basic flow of events and improve the structure of the use case and the use-case model.
You can model extracted parts as:
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An alternative flow of events within the base use case if it is a simple variant, option, or exception
to the basic flow of events.
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As an explicit inclusion in the base use case (see Guideline: Include-Relationship) if it is something that
you wish to encapsulate so that it can be reused by other use cases.
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As an implicit inclusion in the base use case (see Guideline: Extend-Relationship), if the basic flow of
events of the base use case is complete, that is, has a defined beginning and end. The nature of the
extending flow should be such that you prefer to conceal it in the description of the base use case to
render it less complex.
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A subflow in the basic flow of events, possibly as another option, if none of the above alternatives
applies. For example, in a Maintain Employee Information use case, there may be separate subflows for
adding, deleting and modifying employee information.
You can describe use cases in many styles. As an example we show the basic flow of events of the use case
Administer Order described in three different styles, varying primarily in how formal they are. The first
style, shown in example 1 below, is recommended, because it is easy to
understand, and the order in which things happen is clearly evident. The text is divided into numbered and
named subsections. Numbers are there to make it easy to refer to a subsection. Names of subsections will
let the reader get a quick overview of the flow of events by browsing through the text reading only the
headers.
In example 2 below, the description of the flow of events fails to clarify the
order in which things happen. If you write in this style, you and others might miss important things that
concern the system.
Example 3 below shows a yet another style, which can be useful if you find it
difficult to express the sequence of events clearly. This pseudo-code style is more precise, but the text
is hard to read and absorb for a non-technical person, especially if you want to grasp the flow of events
quickly.
1.1. Start of Use Case
This use case starts when the actor Operator tells the system to create a measurement
order. The system will then retrieve all Network Element actors, their measurement
objects and corresponding measurement functions that are available to this particular
Operator. Available Network Elements are those that are in operation, and that the
Operator has the authority to access. The availability of measurement functions depends
on what has been set up for a particular type of measurement object.
1.2. Configure Measurement Order
The system allows the actor Operator to select which Network Elements to measure and
then shows which measurement objects are available for the selected Network Elements.
The system allows the Operator to select from the measurement objects, and then select
which measurement functions to set up for each measurement object.
The system allows the Operator to enter a textual comment on the measurement order.
The Operator tells the system to complete the measurement order. The system will
respond by generating a unique name for the measurement order and setting up default
values for when, how often, and for how long the measurement should be made. The
default values are unique to each Operator. The system then allows the Operator to edit
these default values.
1.3. Initialize Order
The Operator tells the system to initialize the measurement order. The system will then
record the identity of the creating Operator, the date of creation, and the "Scheduled"
status of the measurement order.
1.4. Use Case Ends
The system confirms initialization of the measurement order to the Operator, and the
measurement order is made available for other actors to view.
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Describing a use case: In this style, the text is easy to read and the flow of events is easy to follow.
Aim for this style in your descriptions.
Orderers can create Orders to collect measurement data from the Network Elements.
The system will assign the Order a unique name as well as default values that indicate
the length and time of the measurement and also how often it is to be repeated. The
Orderer will be able to edit these values.
The Orderer must further specify which measurement function, network element and
measurements objects are applicable. The Orderer can also add a personal comment to the
order.
When the necessary information had been defined, a new Order is created and initialized
with the defined attributes, the name of the creator, and the date of creation. The
status of the order will be set to "scheduled". (Possible values for the status are:
Scheduled, Executing, Completed, Canceled, and Erroneous.)
The user interface is then notified that a new Order has been created and receives a
reference to the new Order so that it can be displayed.
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Describing a use case: This style is readable, but there is no clear flow of events.
'Administrate order' (User identity)
REPEAT
<= 'Show administer order menu' IF ( => 'Creating an Order' (Measurement function,
network element, measurement object)) THEN
The system finds a unique name, default values for when and
how long the measurement should be executed.
<= 'Show order' (Default attributes) REPEAT => 'Edit order' (Attribute to change, New value of attribute)
<= 'Update screen' (New attributes) UNTIL (All attributes are defined) REPEAT IF ( => 'Edit order' (Attribute to change, New value of attribute))
THEN <= 'Update screen' (New attributes) ELSIF ( => 'Save order' (Order identity, Attributes)) THEN
The order is created and initialized in the system with
the defined attributes, the name of the creator,
date of creation and the status 'scheduled'.
<= 'New order created' (The order) ENDIF UNTIL (=> 'Quit')
ENDIF
UNTIL 'Quit administer order'
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Describing a use case: Here the writer has chosen a formal style using pseudocode. This style makes it hard
to quickly grasp the process steps, but can be useful if the flow of events is difficult to capture
precisely.
The complete description of the flow of events of the use case Administer Order, including its alternative
flows, could look as follows:
1. Basic Flow of Events
1.1. Start of Use Case
This use case starts when the actor Operator tells the system to create a measurement order. The system
will then retrieve all Network Element actors, their measurement objects and corresponding measurement
functions that are available to this particular Operator. Available Network Elements are those that are in
operation, and that the Operator has the authority to access. The availability of measurement functions
depends on what has been set up for a particular type of measurement object.
1.2. Configure Measurement Order
The system allows the actor Operator to select which Network Elements to measure and then shows which
measurement objects are available for the selected Network Elements. The system allows the Operator to
select from these measurement objects, and then select which measurement functions to set up for each
measurement object.
The system allows the Operator to enter a textual comment on the measurement order.
The Operator tells the system to complete the measurement order. The system will respond by generating a
unique name for the measurement order and setting up default values for when, how often, and for how long
the measurement should be made. The default values are unique to each Operator. The system then allows the
Operator to edit these default values.
1.3. Initialize Order
The Operator tells the system to initialize the measurement order. The system will then record the identity
of the creating Operator, the date of creation, and the "Scheduled" status of the measurement order.
1.4. Use Case Ends
The system confirms initialization of the measurement order to the Operator, and the measurement order is
made available for other actors to view.
2. Alternative Flows of Events
2.1. No Network Elements Available
If in 1.1, Start of Use Case, it turns out that no Network Elements are available to measure for this
Operator, the system will inform the Operator. The use case then ends.
2.2. No Measurement Functions Available
If in 1.2, Configure Measurement Order, no measurement functions are available for the selected Network
Elements, the system will inform the Operator and allow the Operator to select other Network elements.
2.3. Cancel Measurement Order
The system will allow the Operator to cancel all actions at any point during the execution of the use case.
The system will then return to the state it was in before the use case was started, and end the use case.
In the Special Requirements of a use case, you describe all the requirements on the use case that are not
covered by the flow of events. These are non-functional requirements that will influence the design model.
See also the discussion on non-functional requirements in Guideline: Use-Case Model. You could organize these
requirements in categories such as Usability, Reliability, Performance, and Substitutability, but normally
there are so few of them that such grouping is not particularly value-adding.
Example:
In the Recycling-Machine System, a special requirement of the Return Deposit Items use case could be:
The machine has to be able to recognize deposit items with a reliability of more than 95 percent.
It can be useful to use the notion of precondition and postcondition to clarify how the flow
of events starts and ends. However, only use it if it is perceived as adding value by the audience of the
use case.
A precondition is the state of the system and its surroundings that is required before the use case can be
started. A postcondition is the states the system can be in after the use case has ended.
Consider the following:
-
The states described by pre- or postconditions should be states that the user can observe. "The user
has logged on to the system" or "The user has opened the document" are examples of observable states.
-
A precondition is a constraint on when a use case can start. It is not the event that starts the use
case.
-
A precondition for a use case is not a precondition for only one subflow, although you can define
preconditions and postconditions at the subflow level.
-
A postcondition for a use case should be true regardless of which alternative flows were executed; it
should not be true only for the main flow. If something could fail, you would cover that in the
postcondition by saying "The action is completed, or if something failed, the action is not performed",
rather than just "The action is completed".
-
When you use postconditions together with extend-relationships, you should take care that the extending
use case does not introduce a subflow that violates the postcondition in the base use case.
-
Postconditions can be a powerful tool for describing use cases. You first define what the use case is
supposed to achieve - the postcondition. You can then describe how to reach this condition - the flow
of events needed.
Example:
A precondition for the use case Cash Withdrawal in the ATM machine: The customer has a personally-issued
card that fits in the card reader, has been issued a PIN number, and is registered with the banking system.
A postcondition for the use case Cash Withdrawal in the ATM machine: At the end of the use case, all
account and transaction logs are balanced, communication with the banking system is reinitialized and the
customer has been returned his card.
An extension point opens up the use case to the possibility of an extension. It has a name, and a
list of references to one or more locations within the flow of events of the use case. An extension point
may reference a single location between two behavior steps within the use case. It may also reference a set
of discrete locations.
To use named extension points will help you separate the specification of the behavior of the extending use
case from the internal details of the base use case. The base use case can be modified or rearranged, as
long as the names of the extension points remain the same it will not affect the extending use case. At the
same time, you are not loading down the text describing the flow of events of the base use case with
details of where behavior might be extended into it. See also Guideline: Extend-Relationship.
Example:
In a phone system, the use case Place Call can be extended by the abstract use case Show Caller
Identity. This is an optional service, often referred to as "Caller ID", that may or may not have been
requested by the receiving party. A description of the extension point in the use case Place Call could
look as follows:
Name: Show Identity
Location: After section 1.9 Ring Receiving Party's Telephone.
You may choose to illustrate how a use case relates to actors and other use cases in a use-case diagram (in
unusual cases, more than one diagram), owned by the use case. This is useful if the use case is involved
with many actors, or has relationships to many other use cases. A diagram of this kind is of "local"
character, since it shows the use-case model from the perspective of one use case only and is not intended
to explain any general facts about the whole use-case model. See also Guideline: Use-Case Diagram.
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