Shaft Angle vs Attack Angle
Shaft Angle vs Attach Angle - explanation by Dr. Phil Cheetham
Male tour ranges for the driver are from 8 to 18 degrees of forward lean for shaft face on angle at impact.
We use the numbers to see if the golfer is flipping (or scooping) the club at impact or "handle dragging". For scooping, the club angle will be much less than 8 degrees forward and may even be leaning backward at impact. For handle dragging, the shaft angle would be more than 18 degrees forward.
Attack angle is the angle the club head is traveling at impact. If the club had a rigid shaft, then there should be a relationship between the two, but the shaft is not rigid and changes from club to club, so droop, torque (twist) and lag of the club head will affect the relationship. We do not measure those characteristics of the club.
So just to reemphasize, we measure shaft face on angle at impact just to give an idea of the two basic swing faults of scooping and handle dragging.