The panino con rosetta is named not for a filling but for an absence: the rosetta is a hollow roll, and the hollow is the sandwich. The roll is a Roman bread shaped into five raised petals around a domed centre, baked with a thin, brittle, papery crust and almost no crumb, so that when it is split it opens onto an empty chamber rather than a dense interior. Other rolls are filled by compressing bread against meat; the rosetta is filled by setting a few slices into a cavity that was always meant to hold them. The bread is mostly air and shell, and the whole proposition is that empty space, lined with crackle, waiting to be loaded.
The craft is in respecting how little the shell can carry and how fast it goes. A rosetta shatters under the teeth, which is its appeal, but that same crust loses its snap the moment a wet filling sits against it for long, so the roll is filled lightly and close to eating rather than built ahead and held. The classic load is restrained on purpose: a little mortadella, a few coins of salame, a slice of prosciutto cotto, sometimes nothing more than a wedge of cheese, set into the chamber so the bite is shell, then air, then meat, then shell again. An overstuffed rosetta collapses its own walls and stops being itself; the discipline is to let the hollow stay partly hollow and let the crust do the talking. This is bread-counter food, bought at the forno in the morning and eaten by mid-morning before the crust gives in.
The variations are mostly arguments about what goes into the chamber, and they belong to the fillings rather than to the roll. There is the mortadella version that defines the Roman school lunch, the prosciutto cotto and the cheese builds, and the regional cousins of the roll itself: the Milanese michetta, which is the same hollow-shell idea baked under a different sky, and the broader family of crusted rolls a panino can be built on. Each of those is a distinct bread or a distinct filling with its own logic, and each deserves its own article rather than being crowded in here.