The letters R, S is for the flex. The numbers, 100, 200, 300 indicate the level of R flex or S flex. 100 is slightly softer than 300. Fact is True Temper weight sorts to flex, so the 100 would be slightly lighter as well. So and R100 is a R flex shaft that is slightly (only a few grams) lighter and slightly (only a few cycles) more flexible than a R300.