UMLR — Not clear when to use ExecutionOccurrenceSpecification with ExecutionSpecification

Location:
Page #607, 17.2.3 Semantics, Execution Specifications
Page #619, 17.5.3 Semantics, Execution Occurrence Specifications
Title: Not clear when to use ExecutionOccurrenceSpecification with ExecutionSpecification

Summary:
Page #607, 17.2.3 Semantics, Execution Specifications
"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)."

Page #619, 17.5.3 Semantics, Execution Occurrence Specifications
"A ExecutionOccurrenceSpecification represents, on a lifeline, the start event or the end event of an ExecutionSpecification."

When should ExecutionOccurrenceSpecifications be used as start and finish of ExecutionSpecification, and when should Message/Destruction OccurrenceSpecifications be used instead? The ExecutionOccurrenceSpecification is the only one that has a reference back to ExecutionSpecification. Is there a guideline of when to use one over the other?