There is also a "Spine" # 365, 390, 342...what is THAT?
Stiffer is higher or lower or what? I would think that stiffer would be heavier in GPI, but the Easton chart doesn't follow that.
The spine numbers refer to their deflection. There is a specific test procedure where an arrow is placed on two pivot points and a very specific amount of weight is applied to the middle of the shaft to see how much it bends. The amount that it bends is the deflection rating for that shaft.
Stiffer is lower.
Stiffness does not really have anything to do with actual arrow weight. The wall thickness in relation to the diameter of the shaft dictates spine in large part. For example, if you see a large number in the first two digits of the Easton aluminum rating system and a small number in the second two digits then that normally is a stiff arrow for any given setup....when comparing it to an arrow that might have a smaller number for the first two digits and a larger one for the last two. Examples of this might be a 2413 or a 2117. With a setup that might suggest both of those shafts the 2413 will be a stiffer spine than the 2117 when all else is equal.
Hope that did not confuse you more.
And now for the grand finale...how do you pick the "perfect arrow" for your setup without buying 73 dozen varieties and finding 2 that fit your rig better than the others?
Most of the Easton charts I have seen will recommend at least 3 or 4 different arrow sizes for any given setup combination....each normally with a different spine rating. However, normally one or two of those 3 or 4 choices will be darker in print which means that those are the most commonly used sizes for that specific bow arrow setup combination. If you usually stick with those then you won't go wrong in picking the correct setup.
One word of advice though...if you are going with a perimeter weighted style or similar single cam or a hard dual cam then always opt for the stiffer spine out of the recommended choices.