Looking at the moving pictures showing front and back rear shocks it seems to mean shocks on the front for low grip and rear for high where obviously shocks on the front recommended for high grip and on the rear for low.
After a while I came to understand it means rear shocks on the back generate a higher level of traction than the front but as most of the text speaks about high or low grip traction it seems a bit counterintuitive (same for XB4).