The panino con ciriola is named for the bread and not the filling, which tells you where the attention goes. The ciriola is the short, tapered Roman roll, a stubby length of bread with pointed ends, a crisp thin shell, and a soft, faintly hollow crumb. In Rome it is the default carrier for a quick filled sandwich, and the panino takes its name from it the way a sandwich elsewhere might be named for its meat. The roll is the constant; what goes inside is whatever the norcineria or the bar has that day. Naming the sandwich after the bread is an honest admission that in Rome the roll is the decision and the filling follows it.
The craft is in the roll's structure and what that structure allows. A good ciriola has a shell thin enough to yield in one bite without shattering and a crumb open enough to take a filling without going dense, which makes it suited to dressed and slightly wet contents that a tight bread would squeeze out. It is split along the side, not the top, so the filling sits cradled rather than stacked, and it is filled light because the crumb is airy and an overloaded ciriola tears at the seam. Cured meat sliced thin, a soft cheese, a few oil-dressed vegetables: the roll holds them because it was built to, and the better the roll the less the filling has to do. Quality is decided at the bakery, before anything is cut.
The variations are mostly a question of filling poured into the same shape, each its own treatment rather than a list here. There is the plain salume ciriola of the Roman bar and the version dressed with oil-marinated vegetables; both are the same roll meeting a different counter. The wider family of bread-named Roman and central panini, the rosetta and the michetta among them, works on the same principle that the loaf is the signature, and each deserves its own article rather than being crowded in here.