CHAPTER Modeling Functionality with Use Cases 12 This chapter describes how to model the high-level functionality of a system with use cases. 12.1 OVERVIEW Use cases describe the functionality of a system in terms of how it is used to achieve the goals of its various users. The users of a system are described by actors, which may represent external systems or humans who interact with the system. Actors can be classified using generalization. Use cases can also be classified using generalization, but in addition one use case may include or extend other use cases. Actors must be related to the use cases in which they are participants. The relationships between the system under consideration, its