An extension point refers to a location within a use case where you can insert action sequences from other use cases. An extension point consists of a unique name within a use case and a description of the location within the behavior of the use case. In a use case diagram, extension points are listed in the use case with the heading Extension points (appears as bold text in Together).
To add an extension point to a use case diagram, right-click on the use case element, and choose New > Extension Point from the context menu.