The panino con filone is named for the carrier, not the filling, and that is the point of it. The filone is a long Italian loaf, an extended bâtard with a crisp crust and a fairly open crumb, the everyday bread a baker pulls out by the metre. To call a sandwich a panino con filone is to say the bread is the defining variable: a length cut from the loaf, split, and filled with whatever the local counter holds. The sandwich's identity is the loaf's geometry and crust, and the filling is left deliberately unspecified because it changes with the shop and the day.
The craft is the bread choice working as the sandwich's structure. A filone is longer and slimmer than a round loaf, so it is cut into sections and split lengthwise, giving a long, shallow channel rather than a deep pocket; the filling has to be laid in an even ribbon, not piled, or the bite goes uneven and the bread tears. The crust matters here: a filone has enough crisp shell to hold oil-dressed or fatty fillings without going to mush, but the crumb is open enough to compress to the filling rather than fight it. Because the loaf is the constant, the quality of the sandwich is decided at the bakery and the norcineria, not at the cutting board. It is best filled close to eating, while the crust still has snap, since a filone left dressed and waiting softens along the cut.
The variations are whatever the loaf is filled with: a cured-meat filone with one salume, a cheese one, an oil-and-vegetable build for the longer split. Each is the same loaf carrying a different larder, and each deserves its own article rather than being crowded in here, which is what the rest of this catalog is for.