At a guess, the percentage is how much quicker the rack is than the standard rack - i.e. 8% is 8% quicker, 22% is 22% quicker.
My 8% rack was painted dark green, the 22% was bare ali.
According to the parts CD:
RH Drive, 8%, wide track - green
LH Drive, 8%, wide track - yellow
RH Drive, 22% - silver
LH Drive, 8% - blue
RH Drive, 8% - black
The 22% 'non-wide track' rack is also used for wide track with the addition of spacers.
Mike