The Sandwich Poulet is the plain chicken baguette, the everyday baseline of the boulangerie counter. There is no special cut and no regional cure: cooked chicken, grilled or roasted, sliced or pulled, laid into a split baguette with butter or a thin film of mayonnaise, and that is the sandwich. It exists as the default for someone who wants lunch and does not want ham, and almost everything about it is decided by how the chicken was cooked rather than by what gets added afterward. The region is national, found wherever a baguette is sold.
The craft is the chicken and the moisture, and the two are the same problem. Chicken breast cooked even slightly past done turns dry and stringy, and a dry filling makes a dry sandwich, so the entire build depends on either keeping the meat just-cooked and tender or carrying it with enough fat to compensate. That is the job butter and mayonnaise do here: not flavor so much as lubrication, a layer that lets lean white meat read as moist against the crumb. The baguette has to be fresh for the same reason every baguette sandwich does, a real crust to bite through and a soft interior to hold the slices, because the chicken brings no structure of its own. Salt matters more than it seems to: under-seasoned chicken is the most common failure of this sandwich, and a properly salted bird needs almost nothing else. It is a sandwich best eaten within the hour, before the bread tires and the chicken cools to the flat side of plain.
Variations are the obvious next steps once the base exists, which is why the chicken sandwich branches more than almost any other. Add raw vegetables and a vinaigrette and it becomes the Sandwich Poulet-Crudités; fold the meat with a curried mayonnaise and it becomes the Sandwich Poulet-Curry; build it from a carved rotisserie bird, skin included, and it becomes the Sandwich Poulet Rôti. Each is a small addition to the same plain start. The Sandwich Poulet belongs with the roast-and-sliced builds the catalog groups under Baguette Rôti / Bœuf. Its specific contribution is the unadorned baseline the others are measured against: chicken, bread, fat, salt, and the discipline not to overcook the bird.