Use Case Diagram

A use case diagram is a graphical depiction of a user’s possible interactions with a system.

Note, that use-cases are not intended to have any sequencing between them. If you want to sequence behaviors, you should consider e.g. activity or sequence diagrams.

Diagram Model

Name	Graphical Representation	Description
Actor		Models a role that an external actor or external system can take during an interaction with the system. Actors are always outside the system boundary.
Use Case		Defines a fixed set of actions that are provided by the system. Use cases must have a visible benefit for one or more actors.
Association		An association models a relationship between actors and use cases.
Include		The include relationship supports reuse of use cases. Each time, the including use case is called, the included use case is also executed (similar to the call of a subfunction). It is important to avoid inclusion cycles.
Extend		An existing use case can be extended. Please note the direction of the arrows. It’s not an “extended by” logic!
Condition		An extend relationship can be annotated with a condition. If that condition applies, the corresponding use case is taken into account. To model the annotation, use a note shape in white/grey/black color (not the regular note color as described in Annotations).
Specialization		Generalization defines a relationship between a general and a more specific element. Use cases and actors can be generalized.
Realize		Used to show which classes, components, etc. realize the use case. Usually, this relationship is not modelled in a use case diagram.

Example