Hardwood Plywood Grades

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Question
Can someone explain the difference between "A1" and "AA" plywood? My understanding is plywood is graded on a letter scale of A-D, A being the highest. Grade AA plywood would be grade A on both sides. However, I was recently told grade A1 is the best grade. I am trying to understand where the number ratings come into play.

Forum Responses
(Cabinetmaking Forum)
From contributor D:
Typically those grades would mean:
A1 = an A face and a #1 back
AA = An A face as well as an A back.
It is very uncommon to get both A sides.
Some grading will be according to specie specifics as well.

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From contributor L:
Generally you will find AA as a laid up product; most of the plywood suppliers will have A1 as their top grade.

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From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Plywood grades are (best to worst) A, B, C and D for face or front face. The back grades are (best to worst) 1, 2, 3, and 4. Plywood grading standards are published by the Hardwood Plywood & Veneer Association.

Plywood grades of A1, A2, B1, B2 are a good two sided product and would be used where both sides are going to be seen. The back side (number grade) does have a few more blemishes than the front.

A3, B3, C3 are for plywood that will be seen from one side, but the back side must be clean, although there may be some stain or discoloration on the back, spliced veneer, and so on. One might use this material for the sides of a dresser (who looks inside?), for drawer sides, and shelving.

A4, B4, C4 are a one-side-good product. The back may have open knots, filled knots, bad veneer joints or seams. This material will would work well for drawer bottoms, and similar.