An interaction is a root element of a sequence diagram. It is a unit of behavior that focuses on the observable exchange of information between connectable elements, or properties, of the enclosing classifier. You can add interactions to collaborations to contain the diagram elements that participate in one of the collaboration's interactions.
In sequence diagrams, you can create a new interaction use or select an existing interaction as an interaction use. The advantage of using interaction uses in a sequence diagram is that your diagrams can be smaller and less complex.
As the following figure illustrates, an interaction use is displayed as a frame that crosses one or more lifelines.
The keyword ref in the corner of the interaction use indicates that the interaction is referenced from within another interaction.