high speed is good for drilling, low speed can be good for driving large or long screws, it can also give more control over the speed.
Torque settings should be ignored, they suck, they rarely benefit anything ever, put it in drill mode and leave it forever.
If you need to insure your not overtightening something, you should go on slow speed and feel the resistance. Or you could finish it with a screw driver for more feedback/control, or even a torque screwdriver (VERY rarely necessary).
tighten screws until they are snug. rarely do good things happen when you keep tightening past that point.
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