In UML modeling, a region is a container that holds states,
pseudostates, and transitions. You can add a region to a state machine
or a state. You can use regions to define nested states and transitions.
Before you can add a region to a state machine
or state, you must have a UML modeling project open in the Modeling perspective.
To add regions to state machines and states, in the diagram
editor, right-click a state or state machine; then click .