Use Case Diagram

Describe what a system does from the standpoint of an external observer. The emphasis is on what a system does rather than how.
Displays the relationship among actors and use cases

EXAMPLE : A patient calls the clinic to make an appointment for a yearly check up. The receptionist finds the nearest empty time slot in the appointment book and schedules the appointment for that time slot.

Relationships in Use Cases
There are different kinds of relationships between use cases. Reuse of an existing use case using different types of relationships reduces the overall effort required in defining use cases in a system. A similar reuse established using relationships. Use case relationships can be one of the following:

• Include: When a use case is depicted as using the functionality of another use case in a diagram, this relationship between the use cases is named as an include relationship.

• Extend: In an extend relationship between two use cases, the child use case adds to the existing functionality and characteristics of the parent use case

• Generalizations: A generalization relationship is also a parent-child relationship between use cases. The child use case in the generalization relationship has the underlying business process meaning, but is an enhancement of the parent use case.

Use case diagrams are helpful in three areas.
Determining features (requirements)- New use cases often generate new requirements as the system is analyzed and the design takes shape.
Communicating with clients- Their notational simplicity makes use case diagrams a good way for developers to communicate with clients.
Generating test cases- The collection of scenarios for a use case may suggest a suite of test cases for those scenarios.