Hi rathinamsm,

Statemachine state is a syntactical convenience that facilitates reuse and modularity. It is a shorthand that implies a macro-like expansion by another state machine and is semantically equivalent to a composite state. The state machine that is inserted is called the referenced state machine while the state machine that contains the submachine state is called the containing state machine. The same state machine may be referenced more than once in the context of a single containing state machine. In effect, a submachine state represents a “call” to a state machine “subroutine” with one or more entry and exit points.
The entry and exit points are specified by stub states. SubmachineState is a child of State.

Stub state can appear within a submachine state and represents an actual subvertex contained within the referenced state machine. It can serve as a source or destination of transitions that connect a state vertex in the containing state machine with a subvertex in the referenced state machine.

Reference : OMG-Unified Modeling Language, v1.5 2-148