Talking about by-products.
By-products as such doesnt need to be bad. After all, the mouse contains not just only muscular meat. Heart, lungs, stomach, kidneys, blood, fat, even some bones and cartilage, All this is fine for cats.
The problem is, the law isnt as clear cut about defining by products as for defining muscular meat.
So in practice it is all too easily happen, when by products are mentioned and allowed, it will not be the best human grade by products, but the worst one. they couldnt use for anything else, not even for sausage "hot dog". The left overs will probably be beaks, feets, connective tissues, veins. And also dust and other dirt climbed with blood.
So if you want to be safe of good contains - avoid by products...