1. The throttle barrel has a slot in which the screw is set into. This slot is cut diagonally (helically?) and allows the barrel to move in and out as it rotates. (At least in my engines). Correct me if I'm wrong, but I believe this has something to do with retaining the correct mixture at different throttle openings.
2. It can be set to allow a minimum opening as stated above. Don't bother. Use the Tx.
3. I believe it can be set to keep the throttle wide open (for control-line??)

flianbrian