Chuck flap and flap meat share a word and nothing else. They come from opposite ends of the carcass, eat completely differently, and serve different trades. Confusing them on a quote sheet is one of the classic naming traps in beef, in the same family as the two short ribs and the two skirts.
Chuck flap: IMPS 116G
The chuck flap, IMPS 116G, is the serratus ventralis muscle from the underblade section of the chuck. It is one of the most heavily marbled muscles on the carcass, which is why it is prized in East Asian barbecue trades and why boneless cuts fabricated from this muscle group appear on North American menus under the Denver name. It is a chuck item: its supply and its pricing ride the chuck complex.
Flap meat: IMPS 185A
Flap meat, IMPS 185A, is the obliquus internus abdominis, a thin, coarse grained muscle from the bottom sirloin butt. It is the cut France and much of the US West Coast call bavette. Its home is high heat and a marinade: fajitas, carne asada, steak frites. It is a sirloin item, and it trades with the thin meats, the same broad family as the skirts.
Keeping the quote straight
The word flap does all the damage. A supplier quoting flap without a number could mean either cut, and the two do not price alike. The IMPS number is the fix: 116G is the chuck flap, 185A is the flap meat. If the conversation is about short ribs rather than flaps, that is its own naming trap between the chuck and the plate, and it has its own article here.