A region is an element of the state machine. It belongs to a composite state.
A region has no behavior.
The existence of regions allows orthogonal execution in a composite state.
When a composite state has more than one region, the composite state is an orthogonal state.
Interpretation of and deviation from the UML specification
Region contains all states and all pseudostates.
Regions contain all states and most pseudostates except for: entry point, exit point, deep history, shallow history, which belong to states.