The Extends relationship is drawn between Use Cases on a Use Case diagram. An Extends relationship from Use Case One to Use Case Two signifies that an instance of Use Case Two can include the behavior specified by Use Case One, subject to certain conditions specified in the extension. The Extends relation can optionally be named. System Architect automatically adds an <<Extends>> stereotype to the line when it is drawn.
Condition of the extension
The condition is a textual expression specifying the condition which must be fulfilled if the extension is to take place.
Extension point
The extension point indicates the point in the Use Case where the extension takes place. In System Architect you can specify the Use Case Step after which the extension takes place. To add an Extension Point, click the Choices button and drag-and-drop a Use Case Step into the Extension Point field.
Example
For a Use Case scenario such as Place Order, an extension to it would be a Use Case such as Request Catalog. The Extends line is drawn from the Request Catalog Use Case to the Place Order Use Case. Within the definition dialog of the extends line, you can specify the step at which the extension takes place.
