A StateMachine element is a container for groups of related State elements. You can create sections of a StateMachine diagram, showing the organization of the inter-related State elements, and enclose each section in a StateMachine element. You can also create Regions on a StateMachine element.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.306) states:
A behavior StateMachine comprises one or more Regions, each Region containing a graph (possibly hierarchical) comprising a set of Vertices interconnected by arcs representing Transitions. State machine execution is triggered by appropriate Event occurrences. A particular execution of a StateMachine is represented by a set of valid path traversals through one or more Region graphs, triggered by the dispatching of an Event occurrence that match active Triggers in these graphs. The rules for matching Triggers are described below. In the course of such a traversal, a StateMachine instance may execute a potentially complex sequence of Behaviors associated with the particular elements of the graphs that are being traversed (transition effects, state entry and state exit Behaviors, etc.)