Statecharts can get rather big and complex. Composite states are a way to reduce complexity and thus make statecharts easier to create, comprehend and maintain. A composite state comprises a state machine of its own within a region. The states belonging to such a nested state machine are called substates. Orthogonal states are a generalization of composite states, comprising two or more independent state machines in separate regions that are executed virtually concurrently.
A complementary way to mitigate the size of large statecharts are subdiagrams. A subdiagram externalizes the possibly large region(s) contained by a composite state into a subdiagram. In this case the composite state no longer displays its substates. Instead it is visualized very similarly to a regular state. The only difference is a small icon in its lower right corner, marking it as a composite state and giving access to its internal structure: the subdiagram. This way a composite state consumes much less space and gives the user the opportunity to better envision the overall picture. Section "Using subdiagrams" explains how to work with subdiagrams, how to create them and how to inline them again, if needed.
Composite states resp. subdiagrams can be nested to any depth.
The statechart editor provides various refactorings to support editing these hierarchies.