FOC means forward of center.

It is the balance point of your arrow.

10% FOC means your arrows balancing point is 10% forward of the center of the arrow.

As for the spine selection it depends on a few factors.

You draw weight, draw length and length of arrow you will be shooting.

For carbon arrows they have a bunch of different ways of saying the same thing. For example 5575's and 400's they are bascially the same thing, which is what spine you would need.

400 is the spine deflection of the arrow, it should read .400

The 5575 means draw weight from 55 lbs to 75 lbs.

It is just two different ways of listing spine ratings.

Hope this helped[&:]