Architecting and designing : UML 2.5 : Model Elements : 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
Actor
Association
DataType
Enumeration
Extension
FunctionBehavior
Information Item
Interaction
Interface
Opaque Behavior
Primitive Type
Signal
State Machine
Stereotype
Use Case
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Classifiers