The zep is the Norristown long-roll sandwich, and the thing that separates it from the wider hoagie family is how short and fixed its build is. A hoagie shingles several cured meats and piles on a full dressed system of lettuce, oil, oregano, and more. The zep strips that down to a settled, spare list: salami as the single meat, provolone, cooked onion or sliced raw onion, tomato, and a hit of oil, with no lettuce. That restraint is the identity. The zep is not a hoagie with fewer things on it by accident; it is a regional default that fixed a minimal build and stopped there, so what defines it is the absence of the usual crowd, not the presence of anything unusual.
The craft is in making a short list carry the whole sandwich. With one meat instead of a shingle of several, the salami has to be the dominant flavor, so it is the load the rest is balanced against rather than one note among many. The onion is the structural counter that lettuce would normally provide, set in among the salami and provolone for a sharp, sometimes softened bite rather than a cool leafy crunch, and the tomato is the moisture risk, placed within the build so it does not flood the crumb. A pour of oil seasons and lubricates the interior the way it does in any long-roll sandwich, here doing more of the work because there is no vinegar-and-lettuce system alongside it. The long roll is the structural component the whole format depends on, with a crust that holds a layered load down its length without folding and an interior soft enough not to fight it. Built to the local default, it holds together cold from end to end, which is the entire reason a regional name sticks to a fixed build.
The variations are mostly the onion question, cooked or raw, and whether anything is allowed to creep in from the fuller hoagie roster. The zep sits inside the sub, hoagie, hero, and grinder family, the same long roll wearing different regional names and rules from city to city. Each of those is its own codified build and deserves its own article rather than being crowded in here.