What is exactly that "S1 switch" on American Fenders ? Is it some kind of coil split or something like that ?

Can it be found on "Standard" American series ?

I know for sure that on my American Standard Tele that the S1 switch is to combine both the neck and bridge pickups in series to act almost like a humbucker. On a Strat im sure its close to the same. Also on the Tele the S1 switch only works if the pickup switch is in the middle position using both pickups. Hope this helps.