It's the position of the initial track in relation to the grip centerline.  If the track is closer to the centerline at the thumb than at the fingers the PAP will be located above the midline.  If it is closer to the centerline at the fingers than at the thumb the PAP will be located below the midline.  If it is the same distance at both fingers and thumb the PAP will be located on the midline.  --  JohnP