Architecting and designing : UML 2.5 : Model Elements : Structured Classifiers : Protocol State Machine
  
Protocol State Machine
A Protocol State Machine is a specialization of behavioral state machine and is used to express usage protocol or lifecycle of a classifier.
In the Protocol State Machine you specify which operations of the classifier can be called in which state and under which condition, thus specifying the allowed call sequences on the classifier’s operations.
See also
Class
Activity
AssociationClass
FunctionBehavior
Interaction
OpaqueBehavior
State Machine
Stereotype
Device
ExecutionEnvironment
Collaboration
Component
Node
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Structured Classifiers