Ok, here goes another newbie question. I bought a used bow last year and got some Aluminum arrows with it, so when I needed to buy some more arrows, I just got the same kind that came with the bow. Easton Aluminum xx75 2117. I did take the bow in and had it fit to my draw length and had some arrows cut to fit my draw. Now for my question, what do the #' s mean on the side of the arrows, 2117,2213,2216,2219,2315.......????? I know to some of you this is probably a stupid question but asking questions is the only way to learn. Thanks in advance.

2117 = 21/64th" outside diameter and 17/1000" wall thickness. I think.