are you talking about wild or domestic discus? For wilds, there are such standards, like the wild greens are graded by the number of spots on the body. For domestic discus, I don't think there is widely accepted standard for the grade. Different breeders have different ways to sort their fish, like grade A AA&AAA, or grade A,B and C, or some other ways. In this case you might want to check with the specific breeder/distributor for more details about the grading scheme.
-Xiaofei