A carcass moves through three stages of subdivision before it gets to a retail meat case or a foodservice operator's freezer. Each stage carries its own vocabulary, and the vocabulary matters because USDA reports, packer order sheets, and buyer specs all reference the same hierarchy.
The first stage is the primal cut. A primal is a major section of the carcass, usually defined by skeletal anatomy: the rib, the loin, the chuck, the round, and the brisket on a beef carcass; the loin, the butt (Boston butt), the ham, the picnic, the belly, and the spareribs on a pork carcass. There are seven primals in standard beef fabrication and six in standard pork fabrication. Yield shares are roughly stable: on beef, the chuck and the round each carry around 25 percent of the carcass weight, the rib and loin together around 18 percent, and the rest divided among brisket, plate, flank, and trim.
The second stage is the sub-primal. A sub-primal is what the packer actually sells in a wholesale box. The chuck primal yields a chuck roll, a chuck flap, a clod, a chuck tender, and various trim. The loin primal yields strip loins, tenderloins, top butts, ball-tips, tri-tips, and bottom sirloin flaps. A typical retail or foodservice buyer transacts at the sub-primal level, not the primal level. The price lines on USDA reports are nearly always sub-primal lines.
The third stage is the retail cut, which happens at the meat plant inside the grocery store or at a further-processing operation. A strip loin sub-primal becomes individual New York strip steaks. A boneless top inside becomes round roasts and steaks. This stage does not show up in USDA wholesale reports.
What the IMPS code does
Every wholesale beef sub-primal carries an IMPS code (Institutional Meat Purchase Specifications), maintained by the USDA Agricultural Marketing Service. The code identifies the cut, the trim spec, and the packaging convention. A "Rib, ribeye, lip-on, bn-in (109E 1)" line on LM_XB403 is the IMPS 109E spec, portion 1: the lip-on bone-in ribeye trimmed to the published 109E standard. The portion number distinguishes weight ranges within the same cut.
For a buyer, the IMPS code is the unambiguous reference. Two different packers calling a cut "ribeye" can deliver materially different products because of trim, fat cap, and cap muscle treatment. Both calling it "109E" delivers the same product because the spec is published and inspectable. Buyers who buy on spec rather than on packer relationship usually write IMPS codes into their contracts and use them for reconciliation when product arrives.
Pork has its own IMPS series in the 400s. A bone-in pork loin is IMPS 410. A boneless loin is IMPS 413. A boneless butt is IMPS 406A. The bellies, picnics, hams, and ribs carry their own numbers. The series is less commonly used in conversation than the beef series because pork buyers tend to specify by trim percentage and packaging more than by IMPS number, but the numbering is there and the underlying logic is the same.
Why it matters in practice
The hierarchy is more than vocabulary. A buyer making sourcing decisions across primals (do I program more loins or more chucks for the next eight weeks?) has to read prices at the primal level. A buyer making sub-primal decisions (do I run lip-on or boneless ribeye?) has to read prices at the sub-primal level. The two reads use different USDA report sections and different decision frameworks, and conflating them produces decisions that do not hold up.
The IMPS code, in particular, is what lets a buyer compare prices across packers and across days without ambiguity. When two reports list slightly different cut names, the IMPS code is the tie-breaker.