ACA is the American Canine Association, another one of the "alternative" registries that were formed when the AKC got stricter and started requiring DNA testing. A lot of breeders can't qualify for the AKC anymore or were suspended.
These alternative registries are often referred to as "paper" registries as that's all you get, a piece of paper. Most take your word for the fact your dogs are purebred and only require two photographs and a statement from a witness. This witness doesn't have to be a vet or any sort of dog expert either, it could be your Uncle Louie for all they care.
AKC is a minimum requirement IMO, but you still must research a breeder very carefully.