All RC4 dimmers have Set buttons for each individual dimmer output. The fourth button is the Set D button.

On a 2-channel device these are Set A and Set B buttons.

A 4-channel device adds Set C and Set D.

The most common use of the Set button is to assign a DMX channel and dimmer curve to the associated DimX dimmer using RC4 OneTouch; Set A is for the Dim A output, Set B for Dim B, and so on.