Use case classes are used to model and represent units of functionality or services provided by a system (or parts of a system: subsystems or classes) to users. Use cases are denoted as ellipses or ovals. They may be enclosed by a system boundary or rectangle labeled with the name of the containing system. They have the following characteristics:
1. Use cases are interactions or dialogs between a system and actors, including the messages exchanged and the actions performed by the system. Use cases may include variants of these sequences, including alternative and exception sequences.
2 Use cases are initiated by actors and may involve the participation of numerous other actors. Use cases should provide value to at least one of the participating actors.
3.Use cases may have extension points that define specific points within an interaction at which other use cases may be inserted.