Use case diagrams model the behavior of a system and help to capture the requirements of the system.
Use case diagrams describe the high-level functions and scope of a system. These diagrams also identify the interactions between the system and its actors. The use cases and actors in use case diagrams describe what the system does and how it is used by the actors, but not how the system operates internally.
Use case diagrams illustrate and define the context and requirements of either an entire system or the important parts of the system. You can model a complex system with a single use case diagram, or create many use case diagrams to model the components of the system. You would typically develop use case diagrams in the early phases of a project and refer to them throughout the development process.
Use case diagrams are helpful in the following situations:
The following topics describe model elements in use case diagrams:
Use cases
A use case describes a function that a system performs to achieve the user’s goal. A use case must yield an observable result that is of value to the user of the system.Actors
An actor represents a role of a user that interacts with the system that you are modeling. The user can be a human user, an organization, a machine, or another external system.Packages
Packages group related model elements of all types, including other packages.Subsystems
In UML models, subsystems are a type of stereotyped component that represent independent, behavioral units in a system. Subsystems are used in class, component, and use case diagrams to represent large-scale components in the system that you are modeling.
Related tasks
Modeling the behavior of a system by using use case diagrams
Creating use case diagrams
Adding use case diagrams to models
Modifying use case diagrams
Specifying the relationships in use case diagrams