The combo is an Italian beef with a length of Italian sausage laid in alongside the beef, and the interesting decision is not adding the sausage but making two proteins share one roll without either one losing. A plain beef is a single wet pile that the bread and jus organize around. A combo is two competing structures in the same loaf: shaved, jus-soaked beef that wants to slump, and a coarse, charred, springy sausage that holds its shape and pushes back. The sandwich works only if the build keeps both legible in the same bite instead of letting the soft one drown the firm one.
The craft is in sequence and proportion. The sausage is grilled or charred first so it has a firm casing and a smoke note the boiled beef never gets, then it is split or left whole and set into the roll as the spine. The thin-sliced beef is piled over and around it, dipped or ladled with jus so the bread carries the seasoned-stock flavor through every bite, and the sausage rides above the worst of the wet so its char survives the soak. That stacking is deliberate: beef on the bottom takes the liquid, sausage on top keeps its bite, and the eater gets tender and springy in the same mouthful rather than two separate sandwiches taking turns. The roll has to be a sturdy length of Italian bread, because it is now carrying more weight and more liquid than a single-protein order, and it still has to be liftable for the last bite. Hot giardiniera or sweet peppers do double duty here, cutting the fat of two meats at once and adding the only crunch in a build that is otherwise soft beef and yielding sausage. The Chicago counter treats the combo as its own order with its own grammar, wet or dry, hot or sweet, because doubling the protein doubles every decision that a plain beef only has to make once.
The combo sits next to the dipped beef, where the entire assembled roll is submerged in jus and saturation is the whole point, and the plain dry build that keeps one protein and stops the soak at the crumb. Each stays inside the wet-roll logic and each has rules of its own that deserve their own articles rather than being crowded in here.