Alan,
If I am not mistaken the regular birch ply you find in places like the BORG are graded the same as other domestic plywoods. Baltic birch ply on the other hand is imported on GOST(???) standards. Here is a description I found for the grading of Baltic birch ply.
Grade B-Select one-piece face veneer is generally light and even in color. Some small, closed pin knots may be allowed, as well as some brown streaks. NO patches or voids are allowed. The surface is well sanded.
Grade BB-One-piece face veneer is generally light and uniform in color. Occasional pin knots and mineral streaking is allowed. ALL open knots and defects other that those allowed in grade B have been cut out and replaced with oval patches. The patch veneer is the same grade and specie, and is typically of the same color. The surface is sanded.
Grade C-This grade allows patches, open knots, and veneer splits. Grade C is not normally sanded.
So I guess the B-BB Baltic birch you were asking about would have one face graded at B and the opposite face graded at BB.
I hope this helps and if I am off base I am sure someone more familiar with the grading can correct me.
D L