Spareribs, St. Louis ribs, and baby backs are three different products from two parts of the hog: spareribs (IMPS 416) come off the belly side, St. Louis style (IMPS 416A) is a sparerib that has been squared up by removing the brisket bone and cartilage flap, and baby backs (IMPS 422, formally pork loin back ribs) come off the loin, up along the spine. Same animal, different bones, different math.
Spareribs and the St. Louis cut
The full sparerib is the whole belly-side rack. It is the cheapest per pound of the three because it carries the most bone, cartilage, and uneven edge. The St. Louis cut takes that same rack and trims it into a rectangle: brisket bone off, flap off, squared ends. The trim work costs yield, so 416A quotes above 416, and the spread between them is the price of having the plant do the knife work instead of your crew. Sparerib prints also carry weight brackets, with light racks (the 3-down bracket, meaning under about three pounds) speccing separately from medium, because rack size drives portioning.
Baby backs
Back ribs are a different cut entirely, taken from where the rib bones meet the spine after the loin is pulled. They are shorter, curvier, meatier between the bones, and they carry the loin's price gravity rather than the belly's. USDA prints bracket them by weight too, with 2.0 pounds and up as the standard heavy bracket. When back rib prices run, look at the loin complex, not the belly; when spares run, it is usually a BBQ-season and belly-side story.
What the packaging terms mean
Quotes and prints tag ribs with pack styles, and they matter for comparison. Vac or 1 pc vac means individually vacuum-sealed racks, the retail-ready and longest-shelf-life pack, which quotes above bulk for the same meat. Combo means a bulk bin, several hundred pounds loose-packed, the further-processor pack. Poly is a poly-lined box between the two. A vac-pack price against a combo price is not a like-for-like spread, it is meat plus packaging against meat, and the packaging premium is real money at retail margins.