The book does a much better job explaining this than I do, but I'll give it a shot.
the training center has twelve floors, one for each district. Each set of tributes gets the number of floor with which their district corresponds. For example, Katniss, Peeta, and Haymitch all had the penthouse because their district twelve. Rue and Thresh and Chaff and Seeder got the floor below that. Cato and Clove got the second floor. Marvel and Glimmer got the first floor. ECT.
Below all that lies where the tributes actually train while in the training center.