The Sandwich Pique-Nique is defined by where it is eaten rather than by what goes in it. It is the sandwich built to be carried somewhere and opened later: packed at home or bought before leaving, wrapped, set in a basket or a bag, and eaten outdoors hours after it was made. The category is set by use, not by recipe. It can be a length of baguette with ham and butter, a crusted loaf with pâté and cornichons, a hard cheese and a sharp leaf, or whatever travels well from the kitchen to the field. The region is national.
The logic is the logic of a sandwich that has to survive a trip. A picnic sandwich is rarely the delicate one, because the delicate one does not last the journey: the build leans toward fillings that hold for a few hours unrefrigerated and bread with a crust sturdy enough to take being wrapped and jostled without going to paste. Butter does double duty, seasoning the bread and sealing the crumb a little against a moist filling so the loaf does not soften through before it is opened. Cured meat, firm cheese, a pâté, a hard-boiled egg, a pickle for an acidic note: these are the picnic register because they keep. A wet salad filling or a barely-set sauce is the wrong choice, not because it tastes worse but because it will not arrive intact. The bread wants a real crust and a tight crumb precisely so that time, the defining condition of the form, works with the sandwich rather than against it. Unlike most sandwiches, this one is not best within minutes of assembly; it is built to be good an hour or two later, which is the entire point.
Variations are organized by the outing rather than by the ingredient. The roadside version leans on whatever keeps longest; the family-basket version runs to several sandwiches packed together and shared; the cured-meat-and-cheese build is the durable default; a sturdier oil-dressed loaf, given the wait, can even improve on the way there. Each is a recognizable turn within the same carried-and-opened-later idea. The Sandwich Pique-Nique belongs with the use-named sandwiches the catalog groups under Regional Specialty Sandwiches. Its specific contribution is a sandwich engineered around time and transport: built not to be eaten now but to still be worth eating when you get there.