The ciabatta sandwich is a sandwich named for its bread because the bread is the variable doing the work. Ciabatta is a flat, slack-dough Italian loaf with a thin crisp crust and an open, irregular, holey crumb, and a sandwich built on it behaves unlike one built on any soft British roll for reasons that are entirely structural. The defining fact of this build is that the crumb is full of large air pockets: those holes catch oil, dressing and the juice of warm fillings and hold them in the bread rather than letting them run out the side, which makes ciabatta the format of choice for Mediterranean fillings that are wet on purpose, roasted peppers, mozzarella, pesto, oil-dressed salami and tomato.
The craft is matching that crumb to a filling it can absorb without disintegrating, and to a crust that can be managed. The open structure that traps oil is also brittle when the loaf is cold and untoasted, so a ciabatta is usually warmed or pressed, which softens the crust enough to bite cleanly and crisps it where it meets a hot iron, turning the brittleness into a virtue rather than a hazard. Because the crumb soaks readily, the dressing is built into it deliberately rather than guarded against: a drizzle of olive oil into the open holes is the point, not a failure, and it carries seasoning through the whole loaf in a way a tight crumb never could. The fillings are chosen to suit a robust bread, sliced cured meats, firm cheeses, grilled vegetables, things with enough character to stand against a loaf that has a real flavour and chew of its own, where a delicate filling would be lost. A plain soft roll asks the filling to bring everything; a ciabatta is a participant.
The variations are the same idea on the rest of the format shelf, where the bread is again the story. A baguette gives a hard crust and a dense chew that resists rather than absorbs; focaccia gives an oilier, flatter, herbed crumb; sourdough brings tang and a tighter structure. Pressed in an iron a ciabatta becomes a panini, the crust lacquered and the filling fused. Each deserves its own article rather than being crowded in here.