The Denver steak, or zabuton, is sourced from the shoulder or chuck primal. It is a unique cut and is a variation on NAMP 116E with the addition of the neck end of the 116D. It can also be described as the extension of the chuck tail flap/underblade.
This area is made up of three muscles, the primary being the serratus ventralis. Also known as the Beef Chuck, Under Blade Roast, this area below the shoulder blade continues to the ribs and backbone. Many cuts from the underblade area tend to be tough and end up as ground beef or stew meat. But the highly marbled Zabuton is a shining exception.
The shoulder muscles get a lot more use than other areas of the animal which makes them tougher, the specific area that yields a zabuton steak is more of a support muscle that is less used and contains a high level of marbling. That makes it a more tender and flavorful steak option. That also requires superior butchery skills and as a result, it is less common at meat counters.