It probably measures the plate voltage, and I bet the missing units on the scale is so they don't have to expressed it in millivolts right on the face of the unit.
a) love the mV joke
b) warmth = more signal goes through the intentionally overdriven low plate voltage tube. cool = more signal goes through the actually useful and half decent sounding solid state circuit, bypassing the tube stage. somewhere between = worst of both worlds.
keep it on clean and enjoy a cool looking and somewhat useful compressor. pretend it's solid state - oh wait, it IS. the tube distortion circuit (it really IS that - literally - this isn't an insult about the design, it's a fact) is annoying sounding.