Below is an excerpt from the USDA web site about the USDA Shelled Egg Grading Service.
The USDA’s grading service is voluntary; egg packers who request it, pay for it…
What Are Egg Grades?
There are three consumer grades for eggs: U.S. Grade AA, A, and B. The grade is determined by the interior quality of the egg and the appearance and condition of the egg shell. Eggs of any quality grade may differ in weight (size).
U.S. Grade AA eggs have whites that are thick and firm; yolks that are high, round, and practically free from defects; and clean, unbroken shells. Grade AA and Grade A eggs are best for frying and poaching where appearance is important, and for any other purpose.
U.S. Grade A eggs have characteristics of Grade AA eggs except that the whites are “reasonably” firm. This is the quality most often sold in stores.
U.S. Grade B eggs have whites that may be thinner and yolks that may be wider and flatter than eggs of higher grades. The shells must be unbroken, but may show slight stains. This quality is seldom found in retail stores because they are usually used to make liquid, frozen, and dried egg products…
As an egg ages, the white becomes thinner and the yolk becomes flatter.
Very few duck farmers (we know of none) actually go through or pay for the USDA grading process. The reason for this is that duck farmers do not have the mass production of typical large scale chicken farms. The good news is that our duck eggs are fresher than most eggs you’ll ever find in a store. You’re getting them directly from the farm. Our animals are treated well and all accepted sanitary procedures are followed.