I think you are referring to the Future Cruise Certificates. Its "up to $200" based on the length of cruise you book. A 7 day cruise nets you $100 On Board Credit (OBC).
Here's the deal. You buy a FCC on board during your cruise for $100. You don't HAVE TO book a cruise, just the certificate. Then within the next 2 years (I think its 2 now, they just recently changed it from 5) you book any cruise you want. You send in the certificate along with your booking number and they apply the $100 to your cruise pay off amount. Then depending on the length of your cruise you can get up to $200 in OBC. So for a 7 day cruise you are ahead by $100. Its really a good deal. If you for some reason can't take a cruise within the certificates life span you can send it in and they will refund your $100. So it really a no risk thing. (Unless you lose the FCC) You can only use one per booking number. And one per passenger. We get two cabins so we get one in my wife's name and one in mine and put one to each cabin. It works out really well. Which reminds me I still need to send mine in for our August cruise.
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