An interaction occurrence represents a subview of the interaction diagram. It contains the objects that this element touches on the diagram (through the use of hyperlinks you can link it to the sequence diagram that represents this subview). This occurrence is started/triggered by a message being passed to it through the Interaction Connector link.
To create an interaction occurrence, right click on an object, and select New > Interaction Occurrence.
To associate the interaction occurrence with its objects, right click on the interaction occurrence, and choose Select Objects. This enlarges the interaction occurrence to touch/cover the objects it contains.
NB: This feature only applies to UML 2.0. To enable the UML 2.0 Support profile, see Enabling UML Profiles.