Usually, any "Animal Byproducts" are all the parts left over after butchering.... feet/hooves, brains, blood, internal organs, sometimes bones, that sort of thing. All must be cooked to a sterilizing temperature during processing. Sometimes, for livestock use, it's just horse-meat, but it can't be labeled as such.... human sensitivities, ya know??
Most places don't want to go to the trouble to clean intestines for additional use, so they are usually dumped, but they can be included as well, IF all the fecal matter has been removed and cleaned.
BTW - Unsell-able scraps and "Animal Byproducts" are what bologna and hot dogs are usually made from as well.
"Animal Proteins" are the scraps of meat and fat that are trimmed from a carcass during the butchering process, and blood. Also seen horse-meat used here as well.
While these byproducts are not regulated as strictly for animal food as for Human Consumption, they ARE regulated, and inspected. Only "X" parts per million of contaminants are allowed in random-test batches. The facilities that process meat /food for animals and livestock have to maintain the same sanitary conditions and handling procedures as the facilities that process for people food. (and yes, I realize that's not saying much sometimes, but at least there are Some Standards.)
HTH,
Kathy