The Add sub-menu enables you to create a diagram to explain or expand on the selected element, or to create a link to another element. 

Elements such as Actors, Classes and Activities can define a large amount of information that can be conveniently represented by or expanded in a child diagram. The Add sub-menu lists options for creating child diagrams of appropriate types. The types listed as options depend on the type of element selected.

Other elements, such as Timing, Exit and History have much more specific functions that do not require expansion. Therefore, the Add sub-menu for these elements only provides the option to create a link to another element, and does not offer options for creating diagrams.

Although the Add sub-menu lists options to create diagrams of specific types, when you select an option the New Diagram dialog displays and you can select another type of diagram if necessary. Therefore, from the Add sub-menu you can actually create a wide range of diagrams including: