behavior diagrams used to describe a set of actions (use cases) that some system or systems (subject) should or can perform in collaboration with one or more external users of the system (actors)
Each use case should provide some observable and valuable result to the actors or other stakeholders of the system
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UML use case diagrams are ideal for:
Representing the goals of system-user interactions
Defining and organizing functional requirements in a system
Specifying the context and requirements of a system
Modeling the basic flow of events in a use case
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Class Diagram
Modeling classes, attributes, operations, and relationships between objects