The New England hot dog is the only regional hot dog defined by its bun rather than its toppings. The frankfurter is unremarkable and the dressing is left open; what makes it a New England dog is the top-split bun, cut on the top instead of the side, with flat exterior faces that get griddled in butter into crisp gold walls. This is the same roll the lobster roll runs on, and across New England it is the default carrier for almost anything in a bun. The defining thing is structural, not decorative: a side-split bun cannot toast its faces, and a top-split one can, so the regional standard is a bun that arrives with a crisp buttered shell and a soft interior no matter what goes inside it.
It works because the toasted split-top solves a problem every loaded dog has. The bun's flat sides are griddled in butter so the fat caramelizes the crumb into a rigid surface, which both adds flavor and braces the bread against the moisture coming from the sausage and its toppings. A side-split bun under the same load goes limp from the outside in; the top-split's toasted walls hold their structure long enough to finish the sandwich. The frankfurter is grilled, steamed, or griddled to the texture the local stand prefers, and because the bun is doing the structural work, the build can stay as spare or as loaded as the place wants without the bread being the failure point. The split-top is the one element that does not move from Maine to Rhode Island; it is the regional grammar, and the toppings are the dialect each spot fills in.
The variations are the same buttered split-top under New England's own roster. The Connecticut style often deep-fries the dog and dresses it with sauerkraut and a spicy brown mustard; the Rhode Island hot wiener runs a fine meat sauce, mustard, onion, and celery salt; a plain griddled dog with mustard keeps the bun and lets it carry a clean build. Each of those is a codified local set of rules on the shared split-top, part of the broad American hot dog family, and they deserve their own articles rather than being crowded in here.