1: (John) How much food should we order for this dinner?
(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.
In Indian sub-continent; it is also used in sarcasm. It has been abbreviated as "Reply Soon; Varna Pitoge". English Translation would be: "Reply Soon; Else you'll be beaten". It is hence consideredthat the sender requires a reply to his/her letter.