Order one of these at a French counter and nobody asks which ham, because the absence of a qualifier is itself the answer. It is ham on French bread, and almost always the ham meant is jambon blanc: the pale, poached, lightly seasoned ham sliced to order off the deli block. Strip the term back and three things define it: a length of fresh baguette, a layer of barely-salted butter spread to the ends of the crumb, and ham cut thin enough to fold rather than stack. That is the whole sandwich, and the reason it carries no qualifier is that none of its parts is doing anything unusual. The qualifier comes later, when the ham is dry-cured or the cheese arrives or the region asserts itself.
What makes the plain version work is restraint paired with freshness. Boiled ham is mild and slightly wet, so it needs a bread with a real crust to push back and a fat to bridge the two: butter does both jobs at once, cushioning the salt and keeping the crumb from going dry against the meat. The ham has to be sliced thin and laid in loose folds rather than flat sheets, because a thick slab turns rubbery in the bite and a flat sheet pulls out of the sandwich whole. Eaten within a short window of assembly it is clean and direct. Left to sit, the bread softens against the ham's moisture and the sandwich loses the contrast it was built on, which is why the good version is best within a few minutes of being made and the bad version is the one wrapped hours earlier.
Variations are essentially the rest of the French ham shelf. Swap the boiled ham for an air-dried one and you have a different sandwich with a different balance; add cheese and you cross into another tradition; press it under heat and it becomes a hot dish with its own name. Each of those swaps earns its own qualifier and its own treatment rather than being folded in here. The plain Sandwich au Jambon is the baseline they all measure against, and it sits closest to the canonical Jambon-Beurre: the same bread, the same butter, the same ham, named without ceremony because it does not need any.