Unified Modeling Language Reference Manual, The, Second Edition

Use Case

A use case is a coherent unit of externally visible functionality provided by a classifier (called the subject) and expressed by sequences of messages exchanged by the subject and one or more actors of the system unit. The purpose of a use case is to define a piece of coherent behavior without revealing the internal structure of the subject. The definition of a use case includes all the behavior it entails—the mainline sequences, different variations on normal behavior, and all the exceptional conditions that can occur with such behavior, together with the desired response. From the user's point of view, these may be abnormal situations. From the system's point of view, they are additional variations that must be described and handled. ...