As the following figure illustrates, an execution specification is displayed as a rectangle that overlaps the dashed line of a lifeline.
Because the execution specification has a duration, it is represented by two event occurrences, the start event occurrence and the finish event occurrence. The top and bottom of the execution specification represent the start and finish of the behavior in the interaction.
Execution specifications are created automatically when you create a synchronous or an asynchronous message. However, you can manually create an execution specification that is not attached to a message. For example, you can add an execution specification to an existing execution specification to create nested execution specifications in a call sequence.
As the following figure illustrates, a nested execution specification is displayed as a rectangle that is offset from and that covers part of an existing execution specification on a lifeline.