An Interaction Occurrence is a reference to an existing Interaction diagram. Interaction Occurrences are visually represented by a frame, with ref in the frame's title space. The diagram name is indicated in the frame contents. To create an Interaction Occurrence, simply open an Interaction diagram and drag another Interaction diagram into its workspace. A dialog displays, providing configuration options.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.0, p. 423) states:
"An InteractionOccurrence refers to an Interaction. The InteractionOccurrence is a shorthand for copying the contents of the referred Interaction where the InteractionOccurrence is. To be accurate the copying must take into account substituting parameters with arguments and connect the formal gates with the actual ones. It is common to want to share portions of an interaction between several other interactions. An InteractionOccurrence enables multiple interactions to reference an interaction that represents a common portion of their specification."