A stub state only appears on a submachine state.
A submachine state represents the invocation of a state
machine defined elsewhere. In the general case, an invoked
state machine can be entered at any of its substates or through
its default (initial) pseudostate. Similarly, it can be exited
from any substate or as a result of the invoked state machine
reaching its final state. The non-default entry and exits are
specified through stub states. In the UML
metamodel, StubState is a child of State.
Every Stub State has a label on the diagram, which
corresponds to the pathname represented by the “Reference
State” attribute of the stub state.
The description of the details panels of a stub state is
almost identical to that of a pseudo state (see
Section 20.11, “Pseudostate” and so is not duplicated
here. The only difference is one additional field:
Drop-down selector. Allows entering the path name
of the reference state.