Extend Relationships

An extend relationship specifies that the behaviors defined in one use case (extension use case) can be inserted into another use case (base use case). The extend relationship signifies that the incorporation of the extension use case is conditional, meaning its execution is dependent on what has happened while executing the base use case. You can define the exact location where the extension use case is inserted in the documentation for the base use case.

For example, an e-commerce system may have a base use case called "Place Online Order" that has an extension use case called "Specify Shipping Instructions." An extend relationship points from the "Specify Shipping Instructions" use case to the "Place Online Order" use case to indicate that the behaviors in the "Specify Shipping Instructions" use case are optional and only occur in certain circumstances.

An extend relationship appears as a dashed line with an arrow pointing from the extension use case to the base use case. The keyword «extend» is attached to the connector.

You can add extend relationships to your model to indicate the following:

• A part of a use case that is optional system behavior

• A subflow that is executed only under certain conditions

• A set of behavior segments, of which one or more may be inserted in a base use case

The extend relationships in a model usually appear in use-case diagrams.

• Adding Extend Relationships
• Include Relationships
• Use Cases

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