...X2 on the excellent read, ...that will give you a great idea on what spine is technically...
What it doesnt really mention is all different arrow manufactures do not relate spine the same
for instance Easton has a rating marked on more common arrows
300
340
400
500
600
which is the most accurate rating...as the 300 is actually 0.300 and the 340 is 0.340 and so on...so it states actual spine...most common would be the 340's in the 65-70 lb bow weight range...the 400 @ 60 LBs and the 300 @ 75 lbs and so on
Carbon Express uses a different but close rating of
150
250
350
450
which are close but not actual ratings..for example on some models the 350 spine arrows are actually 0.347
other brands mark spine like
4565
5575
7595
that rating is for example the 5575 is good for arrows with a bow weight of 55 lbs to 75 lbs...not as an exact or precise rating as others but easier to understand...
Hope that is as clear as mud...lol
Neil