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InteractionFragment (from BasicInteractions , Fragments ) on page 505
An ExecutionSpecification is a specification of the execution of a unit of behavior or action within the Lifeline . The duration
of an ExecutionSpecification is represented by two ExecutionOccurrenceSpecification s, the start ExecutionOccurrenceSpecification
and the finish ExecutionOccurrenceSpecification .
• start : OccurrenceSpecification References the OccurrenceSpecification that designates the start of the Action or Behavior.
• finish: OccurrenceSpecification References the OccurrenceSpecification that designates the finish of the Action or Behavior.
 The startEvent and the finishEvent must be on the same Lifeline .
start.lifeline = finish.lifeline
The trace semantics of Interactions merely see an Execution as the trace <start, finish>. There may be occurrences between
these. Typically the start occurrence and the finish occurrence will represent OccurrenceSpecifications such as a receive
OccurrenceSpecification (of a Message) and the send OccurrenceSpecification (of a reply Message).
ExecutionOccurences are represented as thin rectangles (grey or white) on the lifeline (see
BasicInteractions , Fragments ) on page 511).
We may also represent an ExecutionSpecification by a wider labeled rectangle, where the label usually identifies the action
that was executed. An
example of this can be seen in Figure 14.13 on page 495.
Issue 8329 - reword paragraph
For ExecutionSpecifications that refer to atomic actions such as reading attributes of a Signal (conveyed by the Message),
the Action symbol may be associated with the reception OccurrenceSpecification with a line in order to emphasize that the
whole Action is associated with only one OccurrenceSpecification (and start and finish associations refer to the very same
Overlapping execution occurrences on
the same lifeline are represented by overlapping rectangles as shown in Figure