Beyond the beef, an Italian beef is defined by what the bread is asked to survive. Thinly sliced, seasoned roast beef is the filling, but the sandwich is not really about the meat: it is about the jus, the seasoned beef stock the meat is held in and the roll is plunged into before it reaches the hand. A long Italian roll, sturdy and slightly chewy, is loaded with the sliced beef and then dipped, briefly or completely, into that jus until the bread is saturated. The defining act is the dip. What separates this from a generic roast beef sandwich is that the bread is deliberately compromised: soaked on purpose, eaten right at the point where it would otherwise fall apart.
The Italian beef rewards a particular structural nerve. The beef is roasted and then sliced against the grain as thin as a deli blade will take it, because a thin slice stays tender in a wet pile while a thick one turns to rope. The jus has to carry the whole sandwich, since every bite of the soaked roll tastes of it, so it is seasoned to be the dominant flavor rather than a background note. Against all that softness and salt the sandwich sets a sharp counter: giardiniera, the oil-cured chopped vegetable relish, for the hot order, or sweet roasted peppers for the mild. Those supply the acid and the crunch that the wet beef and bread cannot, and they are structural as much as flavoring. The Chicago counter codified the grammar of the order around exactly this: how wet the roll, how hot the peppers, whether the whole thing goes back into the jus one more time before it is wrapped.
The variations stay inside the wet-roll logic. The dipped order, where the entire sandwich is submerged rather than briefly bathed, is its own discipline of timing and grip. The combo, which adds a length of Italian sausage alongside the beef, is effectively a second sandwich folded into the first. The Italian beef sits in the same family as the Los Angeles French dip, which keeps the jus on the side rather than pre-soaking the roll, and Buffalo's beef on weck, which trades the Italian roll for a salt-and-caraway kummelweck. Each of those is a real sandwich with its own rules and deserves its own article rather than being crowded in here.