After you identify the use cases, you can create use case diagrams to put the use cases in context. Creating a use case diagram involves establishing a system boundary for a set of use cases and defining the lines of communication between a particular actor and a use case.
In the early stages of a development project, use case diagrams describe real-world activities and motivations. You can refine the diagrams in later stages to reflect user interface and design details.
Defining the system boundary determines what is considered external or internal to the system.
An actor represents a role played by an outside object. One object may play several roles and, therefore, is represented by several actors.
A communicates relationship illustrates the participation of the actor in the use case.
A use case is a set of events that occurs when an actor uses a system to complete a process. Normally, a use case is a relatively large process, not an individual step or transaction.