In Unified Modeling Language (UML) sequence diagrams, you can add execution occurrences to lifelines to show the start and finish behaviors or flow of interactions to visually represent interactions for Java™ or Enterprise JavaBeans™ (EJB) applications.
You can nest an execution occurrence to indicate nested behavior in a call sequence. A nested execution occurrence overlaps an existing execution occurrence, and enables you to distinguish exactly where the activity associated with each message starts and finishes.