The panino con pagnotta is named for its bread and nothing else, which is the whole point of it. A pagnotta is the round country loaf, a tall, domed wheel with a thick chewy crust and a dense, open crumb, the everyday bread of much of central and southern Italy before it is cut for anything. To call a sandwich a panino con pagnotta is to say the bread is the defining decision: a wedge or a horizontal slice off a substantial round loaf, filled with whatever the local counter holds. The carrier is the subject here in a way it rarely is, because a pagnotta has enough structure and flavour of its own that the filling is chosen to suit the loaf rather than the other way round.
The craft is the loaf and the restraint that a strong bread demands. A good pagnotta has a crust with real bite and a crumb firm enough to hold a wet or oily filling without collapsing, which is exactly why it carries cured meat, cheese, or a dressed cooked vegetable better than a soft roll would. It is cut thick, because a thin slice off a country loaf goes leathery and loses the chew that is the reason to use it, and it is filled simply: one cured meat or one cheese, perhaps a drizzle of oil, because a dense, characterful bread already brings flavour and would only argue with a busy build. The loaf is the texture and the keeping quality of the sandwich, sturdy enough to be made ahead and still hold its shape, which is part of why it became the working bread it is.
The variations are the whole map of regional fillings put onto the same round loaf: the cured-meat build, the cheese build, the cooked-vegetable build, the version where the pagnotta is split and oven-warmed first. Each is a town's larder on the same country bread, and each deserves its own article rather than being crowded in here.