The Casse-Croûte Parisien is the sandwich category the French use to mean "something quick, on bread, eaten standing up." The phrase translates literally as crust-breaker, and the original sense is closer to a snack than to a meal: a half-baguette pulled from the boulangerie rack at eleven in the morning, split, filled with whatever the boulanger has prepared that day, and eaten on the walk back to the office. In Paris the casse-croûte is rarely a discrete menu item with a fixed recipe. It is more of a service tier, the line of pre-assembled half-baguettes in the bakery case priced a couple of euros below the full lunch sandwich, made for people who do not want to sit down and do not particularly care which filling they end up with.
The half-baguette is the signature constraint. A demi-baguette is roughly half the length of a full one, fits in one hand, finishes in five or six bites, and costs less. The fillings tend toward the resolutely simple end of the boulangerie repertoire: butter and ham, butter and a slice of Emmental, pâté with a smear of cornichon, occasionally rillettes if the bakery has a charcutier upstairs. There is no plating, no garnish, no warm component. The bread is whatever came out of the morning bake, often a couple of hours past peak crust. The whole point is speed at the counter and portability after.
The category overlaps heavily with the broader Parisian sandwich-on-the-go tradition, but the distinguishing trait is the framing. A Jambon-Beurre is a lunch; a Casse-Croûte Parisien is the same sandwich described as a snack, with a smaller portion and a lower price. The line between the two is mostly social. Office workers buying lunch at the counter ask for the sandwich; office workers grabbing something between meetings ask for the casse-croûte. Other non-baguette breads handle the same job under slightly different names, and the broader Pain Garni & Non-Baguette Breads tradition covers the country breads, sourdough, and seeded loaves the same lunchroom uses for its larger sandwiches. The casse-croûte sits at the small, fast, lower-priced end of that family.