Almost all dairy products can be graded, but the service is used most widely for butter, Cheddar cheese, instant nonfat dry milk, and regular nonfat dry milk. Inspectors also grade other cheeses, dry whey, dry buttermilk, and dried and condensed milk.
The grades of U.S. Extra and U.S. Standard for dry whole milk are based on quality factors like those for other dry dairy products. Grade requirements for dry whole milk also include a maximum bacteria content. Bacteria limits are designed to ensure a safe product that has good keeping quality.
http://www.ams.usda.gov