Difference between Occurrence and Instance in iCalendar standard?

Are there any difference between terms "occurrence" and "instance" in iCalendar RFC 5545?

I found that CalDAV RFC 4791 is using term "instance" only, there is no term "occurrence" used in it.

It looks like these are synonyms or am I wrong?

Solution

My understanding is that (in the context) an 'occurrence' is a point in time calculated by an RRULE.

An 'instance' is an actual event, which would show up in the calendar. Either an instance produced by a recurring event, or an individual event.

Not every 'occurrence' of a recurring event has to end up in an instance. Usually because individual instances got deleted, but the RFC seems to suggest other, timezone related situations in which occurrences get dropped.

Sample:

DTSTART:20160107
RRULE;FREQ=DAILY;COUNT=5
EXDATE:20160109

Presumably 2016-01-09 would be called an occurrence of the RRULE, but it is not an actual instance (5 occurrences, 4 instances).

But as Marten says it is probably not very relevant and potentially not even used consistently in the RFC ;-> Maybe you could explain the point of your question, is there something more to it?