T pool on Millenium class ships is a large heated swimming pool with some jets of water. It is in a public area and access is not limited. T pool on Century Class ships is located inside the Spa. It is small, and much more like a private spa experience. On some cruises you can find 50 people in the T pool on a Millenium class ship. On Century , 5 or 6 would be a lot.
Perhaps they now add a tip for spa personnel handing out towels and locker keys. They were not doing that 3 months ago on Century.