Use Cases are meant to represent the high level functional areas of the system, as represented in a ULM Use Case Model (or use case diagram).  Primarily meant to analyze the different parts of the system and their relationships and dependencies.

Each 'textal' use case is then blown into its own 'user manual' style document detailing the dialogue between the 'system' (the software you are developing) and the 'actors' (people, things or other software that interact with your software).

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Include and Extend are two key constructs in UML Use Case Diagrams. Learn about the two constructs, the differences and how to use them in your model!