Use Case Diagram

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

Describe 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

You generate Use Case diagram elements and connectors from the Use Case pages of the Toolbox.

Example Diagram

This diagram illustrates some features of Use Case diagrams:

Use Case Diagram Element Toolbox Icons

Icon	Description	See also
	An Actor is a user of the system; user can mean a human user, a machine, or even another system or subsystem in the model.	Actor
	A Use Case is a UML modeling element that describes how a user of the proposed system interacts with the system to perform a discrete unit of work.	Use Case
	A Test Case is a stereotyped Use Case element which enables you to give greater visibility to tests.	Test Case
	A Collaboration defines a set of cooperating roles and their connectors.	Collaboration
	A Collaboration Use element allows for a Pattern defined by a Collaboration to applied to a specific situation.	Collaboration Use
	A System Boundary element is a non-UML element used to define conceptual boundaries.	Boundary
	Packages are used to organize your project contents, but when added onto a diagram they can be use for structural or relational depictions.	Package

Use Case Diagram Connector Toolbox Icons

Icon	Description	See also
	A Use relationship indicates that one element requires another to perform some interaction.	Use
	An Association implies that two model elements have a relationship, usually implemented as an instance variable in one or both Classes.	Associate
	A Generalization is used to indicate inheritance.	Generalize
	An Include connection indicates that the source element includes the functionality of the target element.	Include
	An Extend connector is used to indicate that an element extends the behavior of another.	Extend
	A Realizes connector represents that the source object implements or Realizes its destination object.	Realize
	An Invokes connector indicates that source object, at some point, causes the destination object to happen.	Use Case Group
	A Precedes connector indicates that the source object must be completed before the destination object can begin.

Notes

Invokes and Precedes relationships are defined by the Open Modeling Language (OML); they are stereotyped Dependency relationships
Invokes indicates that Use Case A, at some point, causes Use Case B to happen
Precedes indicates that Use Case C must complete before Use Case D can begin

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