The defining tension in a Sandwich au Crabe is freshness against time. Crab is one of the most perishable proteins a sandwich can carry: picked meat loses its sweetness and turns stringy fast, which sets the whole discipline of the build. The meat is dressed lightly, bound with just enough mayonnaise or crème fraîche to hold it together, seasoned with lemon and pepper, and eaten soon. This version reaches Brittany and Normandy alike, where the cold-water catch is firm enough to keep its lumps if it is handled gently and not over-mixed.
The bread does little except hold the meat without crushing it. A split crusted loaf or a soft buttered roll works; the crab is mounded loose rather than packed, so the texture survives to the bite. Acid and salt do the lifting, because sweet crab goes flat without them. There is no warm component and no reason to wait: this is a sandwich made close to service and eaten plainly, the work already done by whoever ran the pots.
The Sandwich au Crabe sits with the cold-water fish builds the catalog groups under Baguette Poisson. Its specific contribution is perishability as a design constraint: a sandwich whose quality is decided almost entirely by how recently the meat was picked.