The Characteristics of Use Case Diagram
Use cases describe the information in the system from the perspective of the users of the system, i.e. the functionality of the system as seen from outside the system, without regard to the specific implementation of that functionality inside the system
A use case describes a visible requirement that corresponds to a specific user goal.
Use cases can be used to delineate the boundaries between the system and external entities.
Use cases are usually executed by an actor.
The use case returns the results of the execution to the actor(s).
A use case is functionally complete. It takes input from the actor and outputs the result to the actor.
