Steve: Not only do the tracks have to be powered, which is done through the 4 channels, but the electronics inside the TIU needs to be powered, continuously. MTH had provided two ways for this to be accomplised. The first, and best, is through the AUX port; any 12-18-volt power supply of at least one amp will do. An alternative method provided by MTH is that, if nothing is plugged into AUX, the TIU wll seek to obtain power from the FIXED 1 input. Problem with this is that, if the transformer handle is pulled back, the TIU shuts off and you lose control on all channels. Therefore, the use of a separate power supply---not your train transformer---feeding AUX is highly recommended.
There are 4 channels to provide greater power and signal strength for larger layouts, and to provide two vehicles---VARIABLE 1 & 2---for using the remote to provide control of track voltage when running conventional. This gives walkaround control when running conventional. I never have to touch a transformer handle
You may feed multiple inputs from a single transformer "handle" or you may use separate "handles" or transformers to provide more power. I have a large layout, capable of running many engines at once, and use 6 channels (2 TIUs) I also have an MTH product, no longer made, that controls voltage output of a Z4000. The feed that through FIXED 1 & 2 (one handle each) and feed the other 4 VARIABLE channels from an OLD ZW and a PH-180. The result is that I can, from the remote, control the voltage on all 6 channels, separately, if I want to run my ancient conventional locos, Having 25 DCS locos, I usually have all 6 channels fixed at about 16 volts.