Use Case Models

A use case is a sequence of transactions performed by a system in response to a triggering event initiated by an actor to the system.

Use cases are normally developed early at the enterprise level and refined at the conceptual level.

They are typically used to specify or characterize the functionality and behaviour of a whole application system interacting with one or more external actors.

The use case model defines the system behaviour in terms of actors, use cases, and interactions between them.  It is an external view of the system.  A full use case should provide a measurable value to an actor when the actor is performing a certain task.

At the enterprise level, only the major system behaviour for the whole organization is modelled.  A subset of these use cases are then expanded at the conceptual level to capture the behaviour for the specific system being analyzed.

A scenario is an instance of a use case.  A scenario can be graphically depicted with a collaboration diagram, also known as an object message diagram, or a sequence diagram, also known as a message trace diagram.  Scenarios are usually developed at the logical level.