UML14 — UML RTF 1.4 Issue: Deferred event ambiguity

What happens when a event is defered in one region, but not another? Is
it left on the queue accessible to both regions, even if it has already
been consumed by one of the regions? Semantics says deferred events are
kept if not used in one of the regions. So if one region uses it, it is
lost, even if it is deferred in the other region. User cannot use event
in both regions.

Reference manual says:

At the time that an object processes an event, it may be in one or more
concurrent states. Each state receives a separate copy of the event and
acts on it independently. Transitions in concurrent states fire
independently. One substate can change without affecting the others,
except on a fork or join caused by a complex transition (described
later).[p 443, Reference Manual]

and refers to an internal queue of events:

Deferred events. A list of events whose occurrence in the state
is postponed until a state in which they are not deferred becomes
active, at which time they occur and may trigger transitions in
that state as if they had just occurred. The implementation of
such deferred events would involve an internal queue of
events. [p 438]

declined