It's an internal code for a room-type bucket. At resorts with reservable "view types", the Unit code sometimes tells you what view you have, but there are no views at SSR, so it doesn't have much use to you in practice.
I don't know if the code has any meaning but as Brian points out, there are no booking categories. The only question is lockoff vs not at this point. Decide on your requests, maybe 1 or 2, and be very specific but not any more limiting than you need to be. Something like a request to be in either the Grandstands or Congress Park, for example.