I shoot Easton arrows. These arrows are normally labeled with "500" or "300" etc. to tell you the spine of them.
However, I've recognized a few brands which label their arrows like "3545" (or something). Note: I can't really tell if this is an accurate number but the formatting is always like this (4 digits which seem to be completely random in my eyes :) ).
So, what does the second annotation mean? What's the difference to the "common" declaration of the spine value? (And why on earth do we need both of them?)