The difference between a strap-on and a strap-off pork loin is a flap of meat and fat, not the fat cap: the strap is the ventral flap left on the belly side of the loin when the two are separated, wide at the shoulder end and tapering away to nothing at the sirloin end. Strap-on means it ships still attached; strap-off means the fabricator removed it, leaving the tighter main-muscle loin that cuts a cleaner chop. On USDA's pork cutout reports you'll see boneless loin lines split into these two fabrication tiers.
What "the strap" actually is
A pork loin runs along the spine from the shoulder back to the hip, and the belly hangs beneath it. When the fabricator separates loin from belly, the knife line leaves a flap of meat and fat on the loin's belly edge. That flap is the strap: wide at the shoulder end, narrowing along the length of the loin, and gone entirely by the sirloin end. USDA's own purchase-option vocabulary names the removal directly ("belly strap off"). The loin eye itself, the longissimus, sits untouched either way; the strap question is about what rides along on the belly edge.
When a boneless pork loin is fabricated, the packer has a choice. Leave the strap attached and chops cut from the shoulder and center sections carry an extra tail of flank-edge meat and fat along the belly side of the chop. Remove the strap and the chop face is a trimmer, more uniform cut with the belly edge squared away. The removal takes seconds per loin and costs a small yield loss (the strap meat rolls into trim).
Why the chop face matters
Premium chop programs spec strap-off by name. A white-tablecloth restaurant slicing chops off a strap-off loin gets a uniform plate presentation: every chop the same trim, no ragged belly-edge tail. The same chop sliced off a strap-on loin carries the extra edge of flank meat and fat, which reads as a lower-grade fabrication even though the actual eating quality is comparable. The bottleneck is plate aesthetics, not anything you can taste.
Commodity retail doesn't care about the seam. A grocery chop in a styrofoam tray is bought on price per pound and the consumer doesn't notice the muscle layout. Mass retail chop programs spec strap-on for that reason: the per-pound discount to strap-off is real and the slicer-face premium is invisible at the point of sale.
IMPS reference
The strap status rides as a purchase option on the spec rather than as a separate IMPS number, and the USDA spec spells the trade names out directly. Both fabrications sit on IMPS 413, the whole boneless pork loin. Purchase option 1 removes the sirloin, giving the short cut. The IMPS 400 series then states that when the loin is prepared that way "and PSO 3 (belly strap off), the item may also be referred to as 'Loin, Short-Cut Back, Main Muscle' or 'Loin, Canadian Back'."
That is the whole vocabulary in one sentence. PSO 1 on its own, with the strap left on, is the short-cut back: what Canadian buyers call a boneless loin short cut. Add PSO 3 and the strap comes off, which is what the Canadian trade calls main muscle. The center-cut boneless loin (IMPS 412B) is a separate, shorter item and should not be quoted against either of these. Wholesale spec sheets from major packers carry the IMPS code and the strap status as separate line items because the two fabrications are separately contracted SKUs with different price sheets. The full IMPS 400 series document is published by USDA AMS and is the authoritative reference for the fabrication standard.
The spread reads as a substitution signal
Strap-off trades at a premium to strap-on. The size of the premium varies by season and by the relative pull from restaurant chop programs vs commodity retail, which is what makes the spread informative. Three readable states:
When both tiers firm together, the whole center-cut complex is tight, chop demand is broad-based across the spectrum from grocery features to restaurant programs.
When strap-off firms but strap-on doesn't, restaurant chop programs are doing the buying. Premium chop fabricators are accumulating cover for foodservice contracts; commodity retail isn't bidding. This is a foodservice-led signal.
When strap-on firms but strap-off doesn't, the opposite: grocery chains are running chop ads aggressive enough to pull commodity center-cut harder than the premium tier. Mass-retail-led. The spread narrows because strap-on rises into strap-off without strap-off moving.
A widening strap-off-to-strap-on spread is a heads-up that restaurant chop programs are getting expensive faster than mass retail, useful for foodservice operators tracking forward cost. A narrowing spread is the reverse: cheap premium spec is signaling restaurant chop demand has cooled.
Practical takeaways for the buyer
If you're a restaurant or foodservice operator specifying chops on a menu, strap-off is the realistic spec, the plate-face premium is what you're paying for. Track strap-off specifically rather than the broader center-cut composite.
If you're a grocery or club-store buyer running chop ads, strap-on is the spec to track and the price you'll actually pay. Watching the strap-off line will mislead you about your own cost.
If you're a price analyst or a grinder buying loins for further processing, the spread between the two tiers is the substitution mechanic to watch. It tells you which channel, premium chop or commodity retail, is doing the buying in any given week, which is information neither tier alone carries.
The Whole Boneless Loin Strap-On spec on USDA reports is the equivalent commodity-tier line for the whole loin (blade end + center cut + sirloin end), not just the center cut. It trades cheaper than CC strap-on because it includes the less-uniform end pieces; the same strap-on vs strap-off logic applies to its center-cut-only sibling.