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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →A use case diagram is a high-level UML view of how external actors interact with a modeled subject to achieve goals. It shows the subject’s externally visible services—not its internal code or a step-by-step screen flow.
What does a use case diagram show?
It depicts a system or other modeled subject from the perspective of its environment: who or what interacts with it, and which goals or services those interactions involve. The Object Management Group’s SysML Version 1.0 describes it as usage of a system by its actors to achieve a goal through services provided to selected actors. The OMG SysML specification provides that foundational definition.
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A use case is generally a meaningful service or process, rather than one button press or transaction. Microsoft Support puts it this way: “Normally, a use case is a relatively large process, not an individual step or transaction.” See Microsoft’s guidance on creating a UML use case diagram.
What are actors, use cases, and the system boundary?
Actors are external roles
An actor represents a role outside the modeled subject. It might be a person, an external system, or another entity in the subject’s environment. “Actor” therefore does not mean only a human end user.
Use cases are services or goals
Use cases represent capabilities or services the subject provides through interaction with actors. Name them with clear goal-oriented phrases, such as “Place order” or “Check account balance,” rather than internal implementation tasks.
The boundary marks the modeled subject
When drawn, a rectangle labels and encloses the subject being modeled. Put use cases inside it and actors outside. This is a scope boundary, not a physical or network perimeter. Microsoft’s Visio instructions likewise place use case shapes within a subsystem boundary and actor shapes outside it.
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What do the lines and relationships mean?
- Association: A line between an actor and a use case indicates participation or communication between them.
- Include: The base use case incorporates another use case’s behavior as part of its execution. It is useful for factoring out behavior shared across use cases; the included behavior is performed as part of the base use case.
- Extend: An extending use case adds behavior to a base use case at a defined extension point, when a condition applies. It represents conditional or optional added behavior, not simply any later step.
- Generalization: A specialized actor or use case is a variant of a more general one and inherits or refines its meaning.
These relationships describe connections among actors and use cases; the precise notation and semantics can vary with the UML version. The cited SysML Version 1.0 specification sets out communication/association, include, extend, and generalization.
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What a use case diagram does not show
A use case diagram does not reveal the subject’s internal structure, code, or detailed order of interactions. The OMG’s UML 1.5 specification says a use case defines behavior “without revealing the internal structure of the entity.” Nor is the diagram a substitute for a detailed textual scenario.
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Use it to communicate scope and actor goals. For the flow of actions or messages, complement it with a textual use case or a behavioral diagram such as an activity, sequence, or state machine diagram. The OMG’s UML overview describes UML as a way to specify, visualize, and document software-system models at different abstraction levels.
How to create a useful use case diagram
- Define the subject. Decide exactly which system, service, or other subject the diagram represents.
- Identify external actors. List the people, external systems, and other environmental roles that interact with that subject.
- Write actor goals as use cases. Focus on complete services or meaningful outcomes, not individual interface actions or internal processing steps.
- Draw the scope boundary. Label the subject, place use cases inside it, and place actors outside it.
- Connect participation. Use associations to show which actors take part in each use case.
- Add relationships only when they clarify meaning. Use include for behavior incorporated into a base use case, extend for condition-dependent additions at extension points, and generalization for variants.
- Move detailed flow elsewhere. Document alternate paths and action order in text or a behavioral model instead of overloading the diagram.
Creating one in Microsoft Visio
Microsoft’s support page documents a Visio workflow: start a UML use case diagram, use the subsystem shape as the boundary, put use case shapes inside and actor shapes outside, then connect them with association or other relevant connectors. The page lists association, dependency, generalization, include, and extend connectors. Visio’s plan and version applicability are described on the support page and may change, so check it for current availability before relying on a particular license.
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