How to use S1, the rotary switch on my Taranis X9 Lite?
I want to use it as a master volume control.
When I go to Special Functions to add it, it isn't an option like the rest of the switches; when you move them they appear as the one you want to assign a function to.
The instructions only point it out as S1, and don't mention it anywhere.

S1 and S2 are not normally switches, they are analog inputs, much like a stick/gimbal.
They can be modified to be a multi-position switch, but do not come that way from the manufacture.
Has yours been modified?

If not: You are probably trying to set the special function as switch, instead use ON for the switch and S1 or S2 for the the parameter.
If you set it as a special function, you would have to set it for every model.

For my use I set it as a Global Function so it works for every model.

(Works the same on a QX7 as an X9D.)

Hasn't been modified.
Stumbled through Open TX to find Global Functions and followed your screen shot.
Thanks...