In UML models, instance specifications are elements that represent
an instance in the modeled system. When you instantiate a classifier in a
model, the instance specification that you create represents an entity in
the modeled system at a point in time, similar to a snapshot of the entity.
You can model changes to the entity over time by creating several instance
specifications, one for each snapshot.
Instance specifications can include the following information about the
entity:
- The classification of the entity by one or more classifiers of which the
entity is an instance
- The kind of instance, based on its classifiers; for example, an instance
specification whose classifier is a class describes an object of that class,
while an instance specification whose classifier is an association describes
a link of that association
- Specific values of the structural features of the entity, which are represented
by slots
Like classifiers, instance specifications have attributes that are referred
to as slots. An instance specification can have one slot for each structural
feature of its classifier, including inherited features. You can specify values
for each slot in an instance specification as long as a valid type is defined
for the slot.
The following figure shows a class named Member. The class has two attributes,
ID and Name. The ID attribute is of type Integer and the Name attribute is
of type UnlimitedNatural.

The following figure shows the corresponding instance specification that
has two slots and values. The value of the ID slot is 1, and the value of
the Name slot is John Q Public.