But I found this article. Overall it's really great!
http://www.vspn.org/Library/Rounds/VSPN_VSPN010821.htm
For all of those who don't want to read the whole article, here is what Elizabeth says about by-products.
What are meat by-products? What are by-products in general? Anything not included in the 'meat' category that is from slaughtered animals. What about hooves and beaks?
Elizabeth Warren: No, those aren't, it's just the clean flesh as described above. Basically what's left when meat or other products are prepared; by-products of the manufacturing of another product. These will include the organs such as the stomach with its contents, the head, feet, etc. But not hair and feathers.