L4--Interlocking ( press ---on. the channel will be off until you press the other one.
Example: It's equivalent to the wind switch of the fan.1-2-3 ,1--gentle wind,2--stronger wind ,3--Strongest wind ,①press 1 ,the 1 on ,2 and 3 off . ②press 2 ,2 on ,1 off ,3 off too , ③press 3 ,2 off ,1 off too