The New England fish sandwich is a frying problem with a regional accent, and the accent is the fish. Where most of the country reaches for generic whitefish or pollock, this sandwich insists on haddock or cod, the cold-water groundfish that the North Atlantic boats actually land, fried and set on a soft bun with tartar sauce. That species choice is the whole identity. Haddock flakes into clean, large, faintly sweet sheets that hold together inside a coating; substitute a softer fish and the sandwich is something else wearing the same bun. The sandwich tastes like the water it comes from, and in New England that water means haddock or cod, not whatever is cheapest.
The craft is in defending the crust against a delicate fillet. The fish is thick enough to stay moist inside its batter or breading but cooked hot and fast so the coating sets and darkens before the flesh overcooks, because haddock turns to mush the moment it is pushed too far. Tartar sauce does double duty: it supplies the acid and fat a lean white fish lacks, and it insulates the crust from the bun rather than soaking it, so the coating stays crisp for more than the first bite. The bun is deliberately soft and unassertive, steamed-tender on purpose, because a crusty roll would fight a fragile fillet and win. The standing tradition of serving it on Fridays is not incidental to the design. It is why the sandwich exists in volume in New England in the first place, a fish meal built to be made fast and eaten without ceremony on a fixed day, which is exactly the constraint that shaped its plain, repeatable build.
The New England fish sandwich sits in the broad American fried-fish family whose founding rule is a mild fillet, a crust that must survive a closed sandwich, and a cold creamy sauce that insulates rather than soaks. Its relatives change the water: the Gulf grouper, the Great Lakes walleye and perch and lake trout, the Southern fried catfish, the standardized chain filet. Each of those deserves its own article rather than being crowded in here.