The panino integrale is named for its bread and nothing else, which is the whole point of it. Integrale means wholemeal: the roll is made with flour that keeps the bran and germ rather than the white refined flour the default panino assumes. Every other Italian sandwich is named for what goes inside, the salume or the cheese or the region; this one is named for the loaf, because the loaf is the variable being declared. The filling is left open precisely so the bread can be the subject, and what the term promises is a denser, nuttier, more assertive crumb than the neutral white roll under most panini.
The craft is in respecting that the bread now has a flavour of its own to manage. A wholemeal crumb is not a blank: it carries a faint bitterness and a roasted, grain-forward note that a delicate filling will simply lose against, so the pairing logic inverts the usual one. A mild, sweet mortadella or a soft fresh cheese tends to vanish here, while a sharper cured meat, a strong aged cheese, or a dressed cooked vegetable meets the bread as an equal rather than being swallowed by it. The texture matters as much as the taste: a good integrale roll is denser and chewier, so it holds an oily or moist filling without going to sponge, but it also tires the bite faster, which argues for restraint in how much goes in. The bread is the lead, and the filling is chosen to answer it rather than the reverse.
Its variations are really a question of what suits a wholemeal crumb. The strong-cheese reading, the sharp-cured-meat reading, the grilled-vegetable build that uses the bread's own savour as part of the balance. The plain white panino farcito is the same act on the default loaf, and the place-named rolls each specify a region instead of a flour. Each of those is a distinct preparation with its own balance to strike, and each deserves its own article rather than being crowded in here.