Counter-Proposals and Offers
When attempting to schedule an event, an attendee may suggest an alternative time by issuing a counter-proposal in the form of a COUNTER message. Upon receiving a counter-proposal, an organiser may then issue an updated invitation with new details (potentially matching those suggested in the counter-proposal, but not necessarily) for attendees to accept or decline. Alternatively, an organiser may refuse to consider other times and send the attendee concerned a DECLINECOUNTER message.
Offers to Schedule
It can be useful for an attendee issuing such a counter-proposal to mark the periods involved as tentatively occupied until they know that their proposal has been accepted or not; it is also useful for the organiser to be able to rely on such periods being temporarily reserved by attendees so that any acceptance of a counter-proposal will succeed instead of failing because an attendee has chosen to dedicate those periods to something else. Thus, an attendee will make a temporary offer of scheduling for periods of time associated with an event, expiring such an offer in case the organiser chooses not to respond in a timely fashion (or at all).
Each attendee invited to an event may respond with a counter-proposal. Thus, an organiser needs to be able to collect multiple counter-proposals for each event and to choose one to accept, if any are to be accepted at all. In principle, an organiser could merge details of different proposals, particularly if multiple periods are involved (and also if different attendees are suggested in different proposals), with the resulting combination of details being issued as a new request to all attendees.