The panino con arancina is carbohydrate on carbohydrate by design: a deep-fried Sicilian rice ball put inside bread. The arancina is already a complete thing, a fist of saffron risotto wrapped around a filling of ragù and peas or ham and mozzarella, breaded and fried to a hard shell. Splitting a roll and pushing one inside is the kind of street excess that should not work and entirely does, the soft bread and the crisp fried crust and the dense rice stacking into a single, deliberately heavy bite. The defining fact is that the filling is itself a finished fried dish, not an ingredient, and the bread is a carrier and a handle rather than a structural partner.
The name is the other defining fact, and it splits the island. In Palermo and the west it is arancina, feminine, after the orange the ball is said to resemble; in Catania and the east it is arancino, masculine, and often more conical than round. The dispute is real and locals hold it firmly. The craft, whichever name is used, is timing and a little crushing: the ball is split or pressed slightly so it seats into the roll, eaten while the shell is still crisp and the rice still hot, because a fried thing that has sat goes leathery and the whole appeal is the contrast of fresh crust against soft bread. The roll is plain and soft on purpose; nothing is added, because the arancina arrived with its own ragù inside it.
The variations are the fillings of the rice ball put into the same roll: the classic al ragù, the al burro with ham and mozzarella, the spinach or the al pistacchio of the eastern towns. Each of those is a different arancina given a handle, and each deserves its own article rather than being crowded in here.