I 've read it this way:
the numbers make increments in 5lbs, the leters make increments in between the numbers in approx 2lbs. there are 3 letters, High,Medium and Low or(heavy, Medium and Light??)
if you have a bow, with lets say Nitrous A cam max out at 60 lbs with the 5h elite limbs, then if you would put 6h elite limbs in that bow you would expect 65 lbs max. if you would take 6M limbs then you can expect 63lbs Max, and if you would take 6L limbs, you would expect 61 Lbs max....
the Nitrous cam has a Draw lenght range of 3", the Martin charts says that there can be a drawweight change of 3 lbs throughout that range, If you need to set the module on the lower end of the range because of the drawlenght you want, then it can be that the bow maxes out a little lower with the limbs according to the the limbcharts ....so if you order a bow with a certain drawweight @ your required drawlenght, than Martin can put in the limbs that will provide your requirements.