Activity Partitions are used to logically organize an Activity. They do not affect the token flow of an Activity diagram, but help structure all or parts of the view.
This example depicts the partitioning between the Classes Process Payment and Order Processor.
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OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.0, p. 12) states:
"A grouping of any set of model elements based on a set of criteria. 1. activity diagram: A grouping of activity nodes and edges. Partitions divide the nodes and edges to constrain and show a view of the contained nodes. Partitions can share contents. They often correspond to organizational units in a business model. They may be used to allocate characteristics or resources among the nodes of an activity."
The OMG UML specification (UML Superstructure Specification, v2.0, p. 307) also states:
"An activity partition is a kind of activity group for identifying actions that have some characteristic in common."