Panels are cut from larger skins of leather such as double shoulders, full sides, full skins, etc, and due to the nature of the item, some variance throughout a skin should be expected.
Grade A panels are what we we describe as near perfect. They will have few blemishes if any, and will be for makers who prefer a more pristine final product.
Grade B panels will have some flaws in the grain. These could be scratches, fat wrinkles, some discoloration, etc. The overall piece will still be usable, but will have some character.