I believe L3 or L4 is the swing index for Head racquets. L4 rated racquets require longer swing than an L3.
I believe Head set it up so that for the longest swing, you can use racquets with L6 rating (Head Prestige Series) and for the shortest swings you would use S12 rated racquets.
It's L6 -> L5 -> L4 -> L3 -> L2 -> L1 -> S1 -> S2 -> . . . -> S12.
Hope that helps.