Used, especially on written invitations, to request a respose.
Used to indicate that a response to an invitation is required or expressly requested.
1: Usually used by the host to have an idea of the number of people attending the occasion.
2: In some cases used for those who are treated with preference.
(Lisa) We can decide that once we get the replies from the RSVP invitations we sent out.
2: Party at exclusive club tomorrow, entry at 10PM. RSVP for free VIP 'guest list' entry. Non RSVP people will have to pay entry fees, no exceptions tolerated.