Short answer
AA: Minimal small live knots. No visible color variation. Uniform grain pattern. Typical use: Premium furniture · Cabinet doors · Visible panels. A grade, labeled AB on a species page: Minor natural knots allowed. Slight color variation acceptable. Clean overall appearance. Typical use: Case furniture · Cabinet bodies · Side panels. B: Knots and color variation allowed. Full natural wood character. Structurally sound. Typical use: Back panels · Non-visible surfaces · Inner parts.
How to order the middle grade
On a species page, choose AB. On the grade card, the same middle face is described as A grade. Write the label you are looking at, plus the cut and thickness, so the quote matches the row you opened.