Screen messages are displayed in the Messages subfolder of a terminal application folder in your service flow project (see Terminal applications subproject). A screen message has the extension .sfmxsd (for example, OperatorIns_1.sfmxsd).
A descriptor is a single recognition criterion for an application screen. Descriptors are of different types. For example, a String descriptor states that an application screen contains a certain text string at a particular location (see Working with screen descriptors).
A screen description is a set of descriptors that together provide a unique description of one state of an application screen. The host editor uses screen descriptions as the basis for recognizing application screens (see Screen recognition). Similarly, when you generate runtime code (see Building a program for a runtime environment), the generated runtime code likewise uses screen descriptions for screen recognition purposes. However, depending on the target runtime environment, the runtime code does not necessarily carry over all the same types of descriptors that are available in the host editor.
A screen message usually contains only one screen description, but it can contain more than one. When an application screen has only one state, then normally you would create for that application screen a screen message that contains just one screen description. The screen description would describe the single state of the application screen.
However, if an application screen has more than one state, then normally you would create for that application screen a screen message that contains multiple screen descriptions, one for each state that you want to recognize. For example, suppose that an application screen that appears after a utility program returns from being invoked has two states that you want to be able to recognize, one state displaying the string "success" and one state displaying the string "failure". You would create one screen message, and in that screen message you would create two screen descriptions, one describing the "success" state of the application screen and one describing the "failure" state.