The trim kit is the finished piece that is mounted on top of the sink (faucet) or on the wall (shower kit). Behind the walls there will be a shower valve where the hot and cold water go in and pipes for the shower head and tub faucet come out. The actual shower head and tub faucet are part of the trim kit. The temperature/volume handle(s) that connect to the shower valve are also part of the trim kit. It is the shower valve, not the trim kit that provides you with the pressure balance or thermostatic control. Changing the trim will only change the look.