Many TAs have room blocks. Usually only verandas, but sometimes Aqua. Depending on the size of the TA anywhere from 25 and up. Some of the larger TAs have 100+ and might work with other companies.
The group rates are to my knowledge very similar to the same and the TAs can turn back an agreed % or number of cabins up to X number of days before the cruise. Have seen 90 days for holidays and as close as 30 days for others.
Those cabin blocks, usually show available on the TA website and booked on the cruiseline site. Am guessing that is what you are trying to figure out.
Full disclosure, I organized group hotel/motel blocks for many years. Others where I worked handled cruise bookings.
Cabin Cruiser is correct about affinity groups. Partially correct about groups assembled by the TA. Just like hotels, cabins can and are put into a block and sold.