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Use Case
The diagram shows the processes within the system,which are visible to the outside world. Basically these are the Use Cases,the Actors of the system being modelled,and the relationships between them.
A use case diagram is a picture of some or all of the actors and their interactions with the system. Each system will have a main use case diagram showing the system boundary and the major functions provided by the system. There will be a communication association between the Actor and the use case.
A simple example use case diagram
