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The main objective of defining requirements in system development is understanding users’ needs, how the business processes are carried out, and how the system will be used to support those business processes. Use Case (UC) modeling helps to discover and understand the requirements for a new system.

A use case is a series of related interactions between a user (AKA “actor”) and a system that enables the user to achieve a goal. In other words, a use case describes the system's behavior as it responds to a series of related requests from an actor. UC models are text documents, NOT diagrams, thus UC modeling is primarily an act of writing text, NOT drawing diagrams. Although UC diagrams are not the primary deliverable of UC modeling, they can be useful because they:

  • show names of use cases and actors and their relationships.

  • illustrate the context of a system and its environment.

  • provide a quick way to list the use cases by name.

UC models are requirements, primarily functional or behavioral, that indicate what the system will do and define a contract of how the system will behave. A use case emphasizes user goals and perspective and answer:

  • Who is using the system?

  • What are their (typical) scenarios of use?

  • What are their goals?