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A UML use case diagram shows how people, other systems, or other external roles interact with a subject to reach goals. It maps the services the subject provides—not its internal code, architecture, or step-by-step screen flow.
What a use case diagram shows
The diagram presents a subject from the perspective of its environment: who or what uses it, and what goals or services those actors interact with. OMG SysML Version 1.0 describes it as showing “the usage of a system (subject) by its actors (environment) to achieve a goal,” with the subject providing services to selected actors. Read the OMG SysML Version 1.0 specification.
Use case diagrams are deliberately high-level. They help communicate the scope and externally visible capabilities of a system; they do not reveal its internal structure. The OMG UML Version 1.5 specification similarly says a use case defines behavior “without revealing the internal structure of the entity.” See the OMG UML Version 1.5 specification.
What are actors, use cases, and the system boundary?
Actors are external roles
An actor represents a role outside the modeled subject. It may be played by a person, another software system, or another entity in the environment. It is not necessarily a specific named individual; for example, “Customer” can describe a role shared by many people.
Use cases describe goals or services
A use case represents a capability or service the subject provides through interaction with an actor. It should generally describe a meaningful goal or process, rather than a tiny interface action. Microsoft Support notes that a use case is normally “a relatively large process, not an individual step or transaction.” Microsoft’s Visio guidance explains use case diagram elements.
The boundary marks the modeled subject
A system boundary, when drawn, names or encloses the subject being described. Put use cases inside it and actors outside it. The boundary indicates modeling scope; it is not a physical or network perimeter.
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Associations connect actors and use cases
An association line indicates that an actor participates in or communicates with a use case. It does not, by itself, provide a detailed sequence of actions or messages.
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What do include, extend, and generalization mean?
These relationships connect use cases or actors in different ways. In particular, include and extend are not interchangeable labels for required and optional steps.
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| Relationship | Meaning | How to think about it |
|---|---|---|
| Include | A base use case incorporates behavior from another use case as part of its execution. | Use it to factor out shared behavior that is performed as part of the base use case. |
| Extend | Another use case adds behavior to a base use case at a defined extension point, under a condition. | Use it for conditional or optional behavior that augments the base use case. |
| Generalization | A more specific actor or use case represents a specialized variant of a more general one. | Use it to show variants that inherit or specialize a common role or behavior. |
OMG SysML Version 1.0 identifies communication/association, include, extend, and generalization among use case relationships. Exact notation details can vary with UML or SysML version, so consult the specification applicable to your project.
What a use case diagram does not show
- Internal implementation: It does not describe classes, code, database design, or component architecture.
- Detailed event order: It does not normally show every action, branch, message, or screen in a process.
- A complete scenario specification: Use case names and relationships provide an overview, not the full narrative of preconditions, steps, alternate paths, and outcomes.
For those details, teams can write textual use case scenarios or use behavioral diagrams such as activity, sequence, or state machine diagrams. UML supports modeling at different abstraction levels; choose the representation that answers the question your audience needs to resolve. OMG’s UML overview describes UML’s modeling purpose.
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How to create a useful use case diagram
- Define the subject and scope. State what system or other subject the diagram covers, and what is outside that scope.
- Identify external roles. List the people, systems, and other environmental entities that interact with the subject. Name roles rather than individual users where possible.
- Write goal-oriented use cases. Name meaningful services from the actor’s point of view, such as “Submit order” or “Track shipment,” rather than low-level clicks or implementation tasks.
- Draw the boundary and place elements. If the boundary is shown, put use case shapes inside it and actor shapes outside. Microsoft documents this workflow in Visio; its page also lists association, dependency, generalization, include, and extend connectors. The available Visio plans or versions may change, so check the page for current applicability.
- Connect actors to the use cases they participate in. Add include, extend, or generalization relationships only when their semantics clarify the model.
- Check the level of detail. Make sure the diagram communicates scope and goals without becoming a screen-by-screen flowchart. Move detailed scenarios or behavior to text or a more suitable behavioral model.
For diagrams created in Visio, Microsoft’s instructions cover placing use case shapes within a subsystem boundary and actor shapes outside it: Create a UML use case diagram.
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