UML Tutorials - Herong's Tutorial Examples - v1.05, by Herong Yang
What Is a Use Case Diagram?
This section describes the UML Use Case Diagram, which presents usage behaviors of a system from an outside user's point of view. Those usage behaviors are organized into units of use cases.
What Is a Use Case Diagram? A Use Case Diagram is a diagram that describes usage behaviors of a system from an outside user's point of view. Those usage behaviors are organized into units of use cases.
In the OMG UML 2.4.1 specification, Use Case Diagram is referred as: Use cases are a means for specifying required usages of a system. Typically, they are used to capture the requirements of a system, that is, what a system is supposed to do. The key concepts associated with use cases are actors, use cases, and the subject. The subject is the system under consideration to which the use cases apply. The users and any other systems that may interact with the subject are represented as actors. Actors always model entities that are outside the system. The required behavior of the subject is specified by one or more use cases, which are defined according to the needs of actors.
The picture below shows you a good example of a Use Case Diagram describing how a restaurant will be used by different types of users:
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