TBA or Category Guarantee simply means the cruise line will assign your cabin according to their needs. They do this in order to evenly dispurse passengers throughout the ship so that all cabin attendants have an equal number of passengers to take care of. The guarantee is that you will be assigned a cabin in the category you booked OR A BETTER CATEGORY. This means that you could be upgraded to a better category but NEVER downgraded to a lessor category. So, if you're not picky about where you cabin is located, you are in a good position to be upgraded. It usually works for me. Have a fun time.
We have been real lucky, so far, when booking like that via RCCL. My only fear is getting a room too far forward or aft. If that is not a concern, go for it. The worse we got was a 4 cat jump up and the best was a 7 cat jump up but please be aware that you may not be upgraded at all.
No idea if it helps but being C and A Diamond members may have an influence on said upgrades.