In sequence diagrams, execution occurrences show the period of time when a participant that is represented by a lifeline is active. Each execution occurrence represents an execution in an interaction.
As the following figure illustrates, an execution occurrence is displayed as a thin rectangle that overlaps the dashed line of a lifeline.
The top and bottom of the execution occurrence represent the start and finish of the behavior that is involved in the communication in the interaction.
Execution occurrences are created automatically when you create a synchronous or an asynchronous message; however, you can manually create an execution occurrence that is not attached to a message. For example, you can add an execution occurrence to an existing execution occurrence to create nested execution occurrences in a call sequence.
As the following figure illustrates, a nested execution occurrence is displayed as a rectangle with a horizontal offset that covers part of an existing execution occurrence on a lifeline.