The grade has to do with orientation of plank relative to the grain of the original balsa log. Actually, this is true of all wood but I digress.
If the plank has long lines running the length of it, it's "A" grain. The plank will flex across the grain.
If the plank has a mottled appearance and shows some lines going crosswise, it's "C" grain (aka "quarter sawn"). The plank will resist flexing across the grain. "C" grain is quite often warped and or skewed as well so the good stuff is hard to find.
If it the plank has some elements of both A and C grain, it's considered "B" grain. It resists flexing both length and width-wise.
For rocket fins I look for the stiffest/densest "B" grain I can find.