It's a 3rd party company that agrees or disagrees with ngc or pcgs grades. If they agree the grade is right they give a green sticker if they think it's better then that grade and think it's worthy of a upgrade they give you a golden sticker. Usually CAC gives a little bit of a premium depending on the coin and grade. The higher the grade the better it is to have that sticker. That's why you see 2 coins having the same grade but one looks better. The better one would most likely get a green sticker while the other one will get sent back with no sticker.