I'm only guessing here, but the most likely functions are either coil-tap or phase inversion.
Select one of the pickups and listen closely for a change in the sound when you move the switch back and forth. If the switch is connected to a coil tap on that pup, the volume will drop, and the sound will get more trebly. Try both pups.
If there doesn't seem to be much happening with just one pup, select both of them, and play some runs with the mystery switch in either position. A phase switch will cause your guitar to sound a bit like a flanging pedal.
If this sorts it out for you, please post back to let us all know what that switch does.
Lordathestrings
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