Well actually (and I hope I don't confuse you saying this) anyone in the cabin can buy one because they are for "Future" bookings only. They can't be applied to the cruise you are on when you buy one. BUT when they are redeemed (i.e. you book a cruise) then only one per booking number can be redeemed AND the person booking the cruise has to be named on the certificate. (So if either you or your spouse wants to be able to redeem it, make you they put it in both your names.)
So lets say you buy one on a cruise. You spend $100. They will deliver it to your cabin before the end of the cruise. Then when you get home you book another cruise. You put your deposit down just like any other booking. Then lets say your balance for your cruise is $1500. You take out your FCC, write your new cruise booking number on it, and mail it to Carnival. When they process it (which could be a couple of weeks before your final payment is due.) They will credit your balance with the $100 so now you only owe $1400. Then they will also give you an on board credit determined by the length of the cruise. A 7-9 day cruise is $100 OBC, $200 for 12 days or more (don't ask me about 10-11 days because I don't know maybe those are $150??) 3-5 days its $50, 6 day is $75. The on board credit should show up on your booking's documents. Then you can spend it on board when you are cruising.
For more details you can look here- What is the Loyalty and Cruise Vacation Program - FAQ's - Carnival Funville
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007