Architecting and designing : UML 2.5 : Model Elements : Classifiers : Opaque Behavior
  
Opaque Behavior
UML specifies several types of behaviors:
Activity
State Machine
Interaction
Opaque Behavior, which has subtype FunctionBehavior
OpaqueBehavior is behavior with implementation-specific semantics, modeled as a place-holder before another behavior is chosen.
See also
Activity
Actor
Association
DataType
Enumeration
Extension
FunctionBehavior
Information Item
Interaction
Interface
Primitive Type
Signal
State Machine
Stereotype
Use Case
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Classifiers