UML24 — UML 2 Events referred to by OccurrenceSpecifications should be optional

In UML 2, every OccurrenceSpecification must be associated with an Event. The Event in turn should have a name and an owning package. But in general, drawing an interaction diagram does not specify a name or owner for the Events associated with the OccurrenceSpecifications.

This means that conforming tools have to invent Events including their names and owners for all OccurrenceSpecifications in an Interaction, even though these objects have no relevance to the model. This simply consumes memory footprint without providing any user value.

If people want to model Events, that is fine; if they wish to associate OccurrenceSpecifications with them that is fine too; what is not fine is forcing the existence of these spurious objects. Hence the multiplicity of OccurrenceSpecification.event and its redefinitions should be changed to 0..1.

The Events that are only defined for Interactions are superfluous and is removed. The necessary information is in the Signal or Operation designated by the signature property of Message together with the MessageKind and MessageSort properties.
In UML 2.3 the signature attribute is derived, in this revision it is changed to non-derived.