I have a '90 300E 3.0L, and on top of the ECU there is what appears to be an adjustment knob with 2 positions indicated (I think it was 0 and 1) on the top of the ECU housing below the knob. What is this for, and where should it be set?

I'm not touching it, seeing a knob on top of an ECU just peaked my curiosity is all......