Panini, as it is eaten in France, is defined by the press rather than by any one filling. A length of soft Italian-style bread, oval and close-crumbed, is split, filled, and clamped between two hot ridged plates until the outside is striped and crisp and the inside is hot through. The filling is whatever the counter runs: cheese that melts, sliced ham, tomato, sometimes a spiced sausage, sometimes a sweet version with chocolate spread. What makes it a panini in the French street vernacular is not the ingredients but the moment under the grill, where the bread loses its softness and the inside fuses into a single hot mass. It is sold from the same windows as the kebab and the croque, made to order, eaten hot in the hand.
The construction works because heat and pressure do most of the assembly. The press flattens the loaf, drives the heat to the centre so the cheese turns molten, and lacquers the bread surface into ridged crisp lines that hold up for the few minutes it takes to eat. The bread has to be soft and slightly absorbent going in, because a crust as firm as a baguette's would resist the press and never bond; the Italian-style loaf compresses instead, which is the whole point. It does not keep. A panini is a hot-now object: minutes after it leaves the grill the crust softens and the inside sets, so it is eaten standing, fast, close to where it was pressed. The cook's job is throughput at the lunch rush, not refinement.
Variations are organised by what melts well under the plates. The plainest French reading is cheese and ham, the cheese chosen for how it flows rather than how it tastes cold. A tomato-and-mozzarella version leans toward its Italian source and goes lighter. A spiced-sausage-and-cheese build is the heavier street option. The sweet version, chocolate spread pressed until it runs, abandons the savoury frame entirely and is sold to the same queue. The constant is the method: soft Italian-style bread, a filling that takes heat, the ridged press, eaten hot and immediately. It belongs to the wider tradition of French sandwiches built on something other than the baguette, covered by Pain Garni & Non-Baguette Breads.