The cochon de lait po' boy is the one filled with pig that was cooked whole and low until it had no structure left to lose. Cochon de lait is suckling pig roasted slowly over a long fire until the meat falls into soft, fatty shreds and the skin renders to a brittle crackling. The defining move is that the crackling is chopped back into the pulled meat rather than discarded, so the filling is two textures at once: collapsed, rich pork and shards of crisp skin distributed through it. That contrast inside a single pile is what separates this from any other shredded-pork sandwich on an airy Louisiana French loaf.
The craft is in the roast and in keeping the crackling from going limp. A suckling pig has thin fat and tender connective tissue, so the slow fire has to render the fat without drying the lean, which is why the cooking is long and the heat patient rather than fierce. The meat is pulled by hand into ropes and shreds and seasoned with a Creole pepper hand; the skin is crisped hard and chopped at the last moment, because rendered crackling surrenders its snap fast once it sits against warm wet meat. The build goes onto the thin-crusted, glass-crackly New Orleans loaf, chosen because it shatters cleanly and stays light under a foot of rich filling without crushing it. Dressed means shredded lettuce, tomato, pickle, and mayonnaise applied as a cool, acidic counter, sometimes with a thin Creole mustard or a vinegar slaw cutting the fat. A festival or po' boy kitchen builds these straight off the roast so the pork is still warm and the chopped skin still snaps.
The variations follow the fire and the dress. A vinegar-slaw build leans the sandwich toward a Carolina logic of acid against fat; a Creole mustard version sharpens it; a smokier roast pushes it toward barbecue. Those readings, like the roast-beef debris, hot sausage, and fried-seafood po' boys in the same family, are their own sandwiches and deserve their own articles rather than being crowded in here.