Architecting and designing : UML 2.5 : Model Elements : Structured Classifiers : Activity
  
Activity
UML specifies several types of behaviors:
Activity
State Machine
Interaction
Opaque Behavior, which has subtype FunctionBehavior
An Activity is a major task that must take place in order to fulfill an operation contract.
An activity can represent:
An entire business process.
A step in a business process.
The invocation of an operation.
Activities can be decomposed into subactivities; the lowest-level Activity is considered a Task or Atomic Action.
Activities and flow of resources between Activities are modeled on Activity Diagrams.
See also
Class
AssociationClass
FunctionBehavior
Interaction
OpaqueBehavior
State Machine
Stereotype
Device
ExecutionEnvironment
Collaboration
Component
Node
Protocol State Machine
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Structured Classifiers