Depending on a cow’s genetics, they’ll produce milk that contains either the A1 or A2 proteins, or a combination of both.
Historically, cows always produced milk that only contained the A2 protein. But with changes in farming demands, most of the milk you see in the supermarkets today contains a mix of A1 and A2 proteins.
For people who have trouble drinking regular cow’s milk, it might be because of the A1 protein. So choosing products made with A2 protein milk only could make all the difference.