The USDA standards for processed, packaged “Salisbury steak” require a minimum content of 65% meat, of which up to 25% can be pork, except if defatted beef or pork is used, the limit is 12% combined. No more than 30% may be fat. Meat byproducts are not permitted; however, beef heart meat is allowed. Extender (bread crumbs, flour, oat flakes, etc.) content is limited to 12%, except isolated soy protein at 6.8% is considered equivalent to 12% of the others.
Wikipedia, Salisbury steak.