In sequence diagrams, you can add execution specifications to lifelines
to show the start and finish behaviors or flow of interactions to visually
represent interactions in applications. Execution specifications represent
the execution of behaviors.
An execution specification is created automatically when you create
a synchronous or an asynchronous message on a lifeline. You can also manually
create an execution specification to show the start and finish behavior in
an interaction.
You can nest an execution specification to indicate nested
behavior in a call sequence. A nested execution specification overlaps an
existing execution specification, and enables you to distinguish exactly where
the activity associated with each message starts and finishes.