The panino con salam d'la duja is built around a salame that never fully dries, because it is not allowed to. In the Piedmontese tradition the sausage is cased, partly cured, and then submerged in melted pork lard inside a tall earthenware jar called the duja, where the fat seals it from air and arrests it in a soft, yielding state. Every other salame gets firm enough to slice into snapping coins; this one stays slack, almost spreadable, closer to a rich paste than a disc. The lard is not packaging incidental to the meat: it is the technique that defines the texture, and the texture is what makes this a different sandwich from the firm-cut salame roll.
The craft is in handling something that will not behave like a normal cured meat. Pulled from the duja, the salame is wiped of its fat coat and either cut thick into soft, almost smearable rounds or worked onto the bread directly, because it has none of the structure that lets a dry salame be shaved. It wants a sturdy carrier with a real crust, a Piedmontese biova or a chewy roll, so that the soft, fat-rich meat has something rigid to be eaten through rather than slumping into a soft crumb. Almost nothing is added: the lard cure has already made it lush and faintly sweet, and butter or oil would only double a richness that is the entire point. It is served at cool room temperature, never fridge-cold, when the preserved fat reads softest and the meat is at its most open.
The variations are best understood by what this is not, and each of those is its own subject. The fine, firm Felino of the Parma hills, the cooked and warm salame cotto of Piedmont itself, the large-grained sweet Varzi of the Oltrepò, the garlic-and-wine mantovano, the fennel-cured Sicilian Sant'Angelo: each is a separate salame with its own grain, cure, and bread, and the soft jar-preserved d'la duja sits apart from all of them. Each of those deserves its own article rather than being crowded in here.