Who is the American Catholic Church? What are the major doctrinal differences between the American Catholic Church and the Roman Catholic church? Why did they split?
According to Wikipedia article on the American Catholic Church of the United States (ACCUS), if that is what you mean, it notes that there are quite a few differences between the ACCUS and the Roman Catholic Church.


ACCUS is post-Vatican II
ACCUS does not require clerical celibacy.
ACCUS clergy members may be married, be in a domestic partnership, divorced, or widowed.
ACCUS clergy members have ordained women among their ranks.
ACCUS does not bar people based on gender, sexual orientation, race, age, marital status, or previous religious affiliations.
ACCUS clergy members are required to support themselves through outside employment; clergy members are prohibited from receiving money for the sacramental services they perform. They are allowed to receive gifts and monetary donations, as long as they are not the ones soliciting them.
ACCUS disagrees with papal infallibility, which was the main reason why they split.


I have a feeling that ACCUS is Catholic in a sense that it observes the seven sacraments, but it disagrees with the Roman Catholic Church on certain sexual-related issues, or at least the article appears to be focused that way. Call it a "liberal" version of the Roman Catholic Church, though any split-off group of the Roman Catholic Church would probably be recognized as Protestant despite the similarity of the theological traditions.