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Regions can be created in Composite States or State Machines on a State Machine diagram. Regions indicate concurrency, such that a single State is active in each region. Multiple transitions can occur from a single event dispatch, so long as similarly triggered transitions are divided by Regions.

To create a Region in a Composite State, follow the steps below:

1.Right-click on a State, and select the Advanced | Define Concurrent Substates menu option. The State Regions dialog displays.
State -- DefineRegionDialog2
2.Construct the Regions of a State, which can be named or anonymous.
3.Click on the OK button.

OMG UML Specification

The OMG UML specification (UML Superstructure Specification, v2.0, p. 476) states:

"A region is an orthogonal part of either a composite state or a state machine. It contains states and transitions."