It is the percentage of tension on the string compared to the draw weight on the bow... I made up a formula in which I did not know if it would go over 100%... I guess it does... The formula is: string tension divided by draw weight times two... This bow DW 48.6 x 2= 97.2 ST 89.7 / 97.2 = 108% Might have to modify my formula if it is going over 100%... It is basically just a reference number that helps me to read the bow better...