A master touch switch controls the lights; these offer full functionality, including turning the lights on/off, dimming, and potentially setting presets. The secondary touch switch is a valuable companion to the master touch switch. It allows you to control the lights from a different location in the room, such as near the doorway. Its design is often simpler, focusing on basic functions like on/off and dimming. For multi-way switching (controlling lights from two or more locations), you'll need both a master and secondary touch switch.