ORIGINAL: ACPOSSETIM
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
This is the right answer on all counts. No reason to et into the technical part of spine testing unless you have a spine tester. If you want to compare a carbon with an alminum the arrow charts usually show a deflection number there for the aluminums. For the record, Easton is the only arrow maker that marks their arrows so we know what the spine is.
And be careful with the Gold Tips, too. They designate their Ultralite series like Easton, but the XT's need to be relabeled. The 5575 is a 400 spine so is way underspined at 75# unless the arrow is about 24" long. Get those 5575's around 29" long or more and they're only good for about 65#. It can get pretty confusing for novices.
Blackhawk is like Carbon Express except that they add another zero. Spines are 2000, 3000, and 4000.
Easton FMJ's are marked like their carbon arrows.