In sequence diagrams, an interaction occurrence is an interaction that is referenced in another interaction frame. You can use interaction occurrences to take common content from one interaction and reuse it in another sequence diagram.
In sequence diagrams, you can create a new interaction occurrence or select an existing interaction as an interaction occurrence. The advantage of using interaction occurrences in a sequence diagram is that your diagrams can be smaller in size and less complex.
As the following figure illustrates, an interaction occurrence is displayed as a frame that crosses one or more lifelines.
The keyword ref in the heading label indicates that the interaction is referenced from within another interaction.