There are two groups of the diagrams in modeling tools developed by No magic Inc. – creatable and not creatable. Only the diagrams of a creatable type can be created (instantiated). A not creatable diagram serves as the base for other types of diagrams.
There are 17 predefined types of diagrams (13 creatable and 4 not). Creatable diagrams are:
Not creatable diagrams:
Communication and Sequence diagrams are subdiagrams of the Interaction diagram. Activity, Interaction, State Machine, and Protocol State Machine diagrams are subdiagrams of the Behavior diagram.
The only way to add a new diagram type to the modeling tool is to extend one of the already existing diagram types and register it. This mechanism is described in Adding new diagram types.