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SysML Use Case Models

A Use Case represents a single unit of meaningful work, providing a high-level view of behavior observable to someone or something outside the system. The SysML notation for a Use Case is an ellipse, as for the UML notation. Use Cases are intended to provide a more detailed expression of the requirements modeled on a higher level.

Use Case diagrams capture Use Cases and the relationships between Actors and the subject (system).  You can use them to:

  • Expand on (or 'realize') the functional requirements of the system
  • Describe the manner in which outside things (Actors) interact at the system boundary
  • Describe the response of the system

Use Cases also support more detailed definition via Use Case Scenarios.

Use Case Diagram

A SysML Use Case diagram is used to define and view Use Cases and the Actors that derive value from the system. The Use Case diagram describes the relationship between the Actors and the Use Cases. Enclosing the Use Case within a Boundary defines the border of the system; the Actors by definition lie outside the boundary. (All elements are internal to the SysML diagram frame.)

While the Use Case diagram can appear simplistic, it is a powerful communication device that describes the value or goals that external roles achieve from interacting with the system. Each Use Case can be detailed, with descriptions, constraints and any number of scenarios that contain sets of steps performed alternately by Actor and system to achieve the desired goal.

Use Case Scenario

A core aspect of modeling Use Cases is performed using the Use Case Scenario feature, which helps add a more detailed text-based representation of the underlying Use Case.

Using the Scenario builder, behavioral model diagrams can then be generated from these text-based details to provide a base for the more detailed design.


The main elements that can appear in Use Case diagrams are:

  • Actor
  • Use Case
  • Boundary

The main connectors that can appear in Use Case diagrams are:

  • Communication Path
  • Generalize
  • Include
  • Extend


A Use Case diagram can be used to define the details of a Use Case and its Scenarios and Constraints, with tight connectivity and traceability, as the Use Case diagram and the textual details of the Use Cases, Scenarios and Constraints are all contained in the same model. From the Use Case scenarios you can generate Activity diagrams, Sequence diagrams and StateMachine diagrams as starting points for the analytical modeling.

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