The submarine sandwich is defined by its bread before anything else: a long soft roll, split along its length and filled end to end, eaten in the hand as one continuous thing. In Britain the sub arrived as an American import and largely stayed in that register, the high-street and chain format rather than a regional one, but the engineering is the same wherever it is made and that engineering is the point. A sliced loaf gives two flat faces and a fixed footprint; a long roll gives a hinged trough. The whole construction follows from that shape, because a deep narrow channel holds a loose, mixed, generous filling in a way two slices pressed flat never can.
The craft is loading a trough so it eats evenly and survives being held. The roll is split most of the way through but left joined along one edge, a hinge rather than two halves, so the filling is contained on three sides and the sandwich does not shed its contents from the bottom. Because every bite has to carry a fair share of meat, cheese, salad, and sauce across a long roll, the fillings are arranged in even lengthwise runs rather than stacked, and the dressing is applied along the bread as a measured stripe so the far end is not dry while the middle goes wet. The roll itself is the structural decision: soft enough to bite cleanly without the filling squeezing out the back, sturdy enough in the crust that a long span does not sag and split under a heavy load. It is built to be eaten moving, which is why the containment matters more than it would on a plate.
The variations are an argument about the roll and the fill rather than departures from the long-roll idea. The cold-cut sub runs layered cured meats and salad the length of the bread; the meatball sub puts a hot, sauced filling through the same trough and leans harder on a roll that can take the moisture; the toasted or melted version crisps the bread and fuses the cheese. The baguette-built and the ciabatta-built versions change the crust and the chew while keeping the format. Each deserves its own article rather than being crowded in here.