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Use cases are a means for specifying required usages of a system. Typically, they are used to capture the requirements of
a system, that is, what a system is supposed to do. The key concepts associated with use cases are actors, use cases, and
the subject. The subject is the system under consideration to which the use cases apply. The users and any other systems that
may interact with the subject are represented as actors. Actor s always model entities that are outside the system. The required
behavior of the subject is specified by one or more use cases, which are defined according to the needs of actors.
Strictly speaking, the term use case refers to a use case type. An instance of a use case refers to an occurrence of the emergent
behavior that conforms to the corresponding use case type. Such instances are often described by interaction specifications.
Use cases, actors, and systems are described using use case diagrams.