I just ordered a replacement seat control switch for the passenger side front seat on my newly acquired 1994 XJ12 (XJ81) and in looking at the old one and the pictures of the replacement switch in several ebay listings I see a rotary dial on the back of the switch. The part number for this switch is DBC11320. The driver's side switch is DBC11321. See photos below:

Front of switch

Back of switch

What is this green rotary dial on the back of the switch?

 

I'd bet it's a backlight bulb (12V, 2W), not a dial. Give it a quarter turn counterclockwise and see if it comes out.

 

Yes, you are correct. This is an access method to replace a miniature bulb inside the switch.
Thanks

 

Speed control, if you turn the dial all the way clockwise, it will launch the occupant out of the sunroof. Be careful as there's one on each seat control. Depending on RHD or LHD, it should be set for the passenger for obvious reasons.