In Philadelphia the meatball sandwich is a hoagie, and the Philadelphia instinct is restraint with the sauce. Where other cities flood the roll and call the result generous, the hoagie tradition treats marinara as a binder and a seasoning, not a soup. Meatballs are set into a long Italian roll with just enough sauce to coat them, provolone is melted over the top, and the build stays controlled enough that you can pick it up and eat it like a hoagie rather than chase it with a fork. The provolone, not mozzarella, is the local signature: a sharper, firmer cheese that holds its own against the meat instead of disappearing into it. That combination of a dry-side build and an assertive cheese is what separates the Philadelphia version from its New England and New York cousins.
The craft is in the roll and the proportion. A Philadelphia hoagie roll has a crust with real structure and a tender interior, the same loaf that carries a cheesesteak, and it is chosen because it can take a moderately wet load without folding at the middle. The meatballs are a beef-and-pork blend, sized to sit two or three across the length of the roll so every bite reaches one, and they are sauced off the heat so the marinara clings rather than runs. The provolone is laid on while the meatballs are still hot and either melts from that heat alone or gets a short pass under the broiler, enough to soften and bind without turning the roll to mush. The restraint is structural as much as stylistic: less sauce means the crust survives, and the eater is rewarded with bread that still has bite at the end.
The variations stay inside the hoagie frame. Sharp provolone gives way to a milder one for a softer build; long hots or sauteed peppers add heat and a vegetal counter; some shops add a dusting of grated hard cheese for salt. The roll occasionally shifts to a seeded loaf depending on the shop's baker. The New England grinder runs the same meatballs back under heat with a toasted roll, the New York hero leans on a mozzarella broil, and the national meatball sub standardizes the whole idea, and each of those deserves its own article rather than being crowded in here.