The Sandwich Poulet-Crudités is defined by the combination, not by either half of it. Crudités are the raw vegetables of the deli case: sliced tomato, cucumber, grated carrot, a leaf or two of lettuce, sometimes a ring of onion. On their own they make a thin, watery sandwich; chicken on its own makes a dry one. Put together in a split baguette with a film of mayonnaise or a vinaigrette, the two failures correct each other, and that mutual correction is the entire reason the sandwich exists. The region is national, a fixture of the boulangerie cold case across the country.
The craft is the logic of the pairing carried through to assembly. Cooked chicken breast is lean and tends dry; raw vegetables are cool and bring moisture and crunch; the dressing bridges them and seasons both. Built in the right order the sandwich works as a single thing rather than two: a thin layer of mayonnaise or vinaigrette against the crumb to waterproof the bread, the chicken laid in, then the vegetables on top so their moisture travels down through the meat rather than straight into the bread. Salting the tomato and draining it first is the difference between a crisp sandwich and a soggy one, the most common failure of the form. The carrot adds sweetness and a fibrous bite, the cucumber a clean cold note, the lettuce structure. The baguette needs a real crust for the usual reason, to brace a soft, moist interior, and the whole thing has a short window: this is a sandwich best eaten within an hour or two, before the vegetables surrender their water and the bread gives up.
Variations are mostly a matter of which raw vegetables go in and what dressing carries them, a mustard-laced mayonnaise in one version, a sharper vinaigrette in another, a hard-boiled egg sliced in for body. Drop the chicken entirely and the same crudités and bread become a vegetable sandwich in their own right. The Sandwich Poulet-Crudités sits next to the plain Sandwich Poulet it builds on, and belongs with the roast-and-sliced builds the catalog groups under Baguette Rôti / Bœuf. Its specific contribution is the combination itself: dry meat and watery vegetables solving each other inside one loaf.