I'm not a 100% on this but I think on carbon arrows it represent thousanths of an inch deflection when the arrow is supported on each end and a calibrated weight is hung from the middle of the arrow. The larger the number the softer the spine (on easton/bemans anyway, however carbon express is different they also have 200/300/400 but the larger the number the stiffer the spine but I'm not sure what the # represents in that scenario) I'm not sure if this also applies to aluminums or not

hope this helps