As far as grading of a flamed top, like AA, AAA, etc. I realize the more A's the nicer looking, more figured, better flame. But does it actually mean anything? Is it something measurable in any way or is it just marketing?
Excellent question!

I always assumed the wood passed through a trained team of eyeballers who determine the grade.
'11 Gibson Buckethead LP
'97 Fender Cali Series Strat
?? Samick Bass
'01 Fernandes Dragonfly Elite
Yes it does. I graded construction grade maple for years, but in grading school saw and was taught how to grade higher end specimins. It basically put starts with the typical color/ imperfection grading then goes into grading how well the respected peice is figured. A well figured peice of flame maple will have a pretty consistant pattern across the entire peice and be a quite distinct difference between the stressed wood and the normal wood for that lovely '3d' effect.
The other question I always have is whether the wood is graded prior to actually carving the top, or if the carve is done first, and then the figuring is judged. Regardless, the degree of "bookmatching" of a two-piece top can't be totally predicted until the top is carved, since the matching of the uncarved two-piece top will change as the carve takes away wood (but it can be minimized if the amount of wood taken off at the seam is minimal.

PRS has regular tops, 10-tops, then Artist-grade tops, and then they get into the "Private Stock" wood. My impression is that the grade is designated before carving, although maybe for the higher-volume regular and 10-top versions they wait and see how the carve turns out... But it does seem to be the case that some non-10-tops seem to be just as nice looking as some of the 10-tops.

The above applies to guitars where the top wood is solid - obviously it is easier to get consistently nice flaming if you are using thin veneers that are glued on.