On some of the carbons it refers to how much they flex in a specific, measurable situation. Jeff and Arthur probably know this one but I believe it has something to do with hanging a specific weight from the the middle of an arrow supported at two spots and at a specific distance apart.

In any case, whether it is the actual amount that they bend or just a number, they are all used to differentiate between the different spine rating for any given brand of carbon arrows. In the case of something like the Carbon Express (100, 200, 300, 400, etc..) the lower number is the weaker spine. In the case of something like the Beman ICS hunters (500, 400, 340, 300, etc...) the higher number is the weaker spine.

Hope this does not confuse you more.