Use Case Diagram :-
Use case diagram is designed for visualizing and documenting the behavior of a system. The main notations of use case diagram include System (subject), Use Case, Actor, Association (communication link), Dependency and Generalization. Use case diagram is usually used in modeling the context of a system and requirements of system.


Elements of a Use Case Diagram:-

  1)Actors :- An actor portrays any entity (or entities) that performs certain roles in a given system.
                  An actor in a use case diagram interacts with a use case

2)Use case:-A use case in a use case diagram is a visual representation of a distinct business functionality in a  
                  system.
3)System boundary: -A system boundary defines the scope of what a system will be. A system cannot have  
                                infinite functionality.
               Use cases share different kinds of relationships. A relationship between two use cases is basically a dependency between the two use cases.