The roast pork sandwich is the cold-pork baseline of the roast cluster, and what defines it is the fat. Cold roast pork is a mild, faintly sweet meat that firms as it cools, and its leftover fat sets into something soft and a little waxy that coats the palate and reads, unrelieved, as one heavy register from the first bite to the last. Every good version of this sandwich is an answer to that fat. The classic answers are a sharp apple sauce and a shard of crackling, and the model description names both, but the plain sandwich is the form before those decisions are made: pork, bread, and whatever single counter the kitchen reaches for. The pork carries the body; the choice of what cuts it is the whole argument.
The craft is in the cut and the moisture. Cold roast pork is at its best sliced thin and against the grain, because a thick cold slice is dense and pushes the set fat forward, while a thin one stays tender and folds to the loaf. Off the joint the meat has lost the heat and the running juices it had on the plate, so the build has to put lubrication and a counter back: a measured smear of apple sauce, a sharp mustard, butter spread to the edges, or a thin layer of the pork's own dripping. The bread wants real structure for a dense, slightly fatty filling, a sturdy white or a sliced bloomer rather than anything soft, and it is buttered to bridge the salt of the pork to the wheat and to keep the crumb firm under the meat. Done plainly and well, this is a sandwich decided by three or four honest things in the right proportion, which is exactly why it works as the baseline the others are measured against.
The variations are the named decisions about how to meet the pork. Apple sauce supplies the sharp-sweet acid that cuts the fat; crackling adds the shattering texture a soft cold filling otherwise lacks; sage and onion stuffing brings a savoury aromatic layer; English mustard takes a hotter, drier route. The hog roast roll built hot off the spit belongs to its own tradition. Each deserves its own article rather than being crowded in here.