You need a controllable light fixture. The d120 is not one though some have managed to hack it but not reliably. The point of a controller is to monitor tank parameters, monitor things like water levels via float switches, control relays that can control your pumps, skimmer, reactors, etc. notify you of any issues and manage overall operations like what to turn off when your feeding or during water changes, they can also control dosing pumps, calcium reactors, etc. I'm not sure the difference but I think it has to do with the screen and the capability of the main module. Most sites have charts showing the difference between models.