A1 casein is in the majority of massed produced cow's milk products. The A1 casein molecule is not only indigestible to many people and the molecule is very similar in structure to both the gluten molecule and also to a molecular structure in thyroid gland tissue, which can lead to antibody cross reactions in people who already have antibodies to these molecules, like people with Celiac Disease and people with Hashimoto's Thyroiditis. Whereas A2 casein is in goat and sheep's milk products and some cow's milk products. The difference between A1 and A2 is one of the reasons many people who are allergic to dairy or consider themselves lactose intolerant but can digest goat's milk or sheep's milk better than cow's milk.
This is why, in ph360, if someone reports being lactose intolerant, their cow's milk smilies drop a lot but goat and sheep a bit less.