The French dip is defined not by the beef but by the cup beside it. Thinly sliced roast beef is piled onto a French roll, and what makes the sandwich itself is that the jus, the seasoned beef stock, is served separately for the eater to dip into bite by bite rather than poured over the whole thing in the kitchen. That single decision is the entire idea. A roast beef sandwich becomes a French dip the moment the moisture is handed to the diner as a control rather than baked in as a default. The bread stays its own structure right up until the eater chooses to compromise it, one dunk at a time.
The craft is in the slicing, the jus, and the roll, in that order. The beef is roasted and then sliced against the grain as thin as the machine allows, so a forkable pile stays tender instead of turning chewy when it meets liquid. The jus is the load-bearing flavor, since it will season every bite the eater dips, so it is built to stand on its own out of a cup rather than to merely moisten. The French roll is chosen for a crust with enough structure to take repeated dipping without collapsing in the hand: dunk too long and it dissolves, dunk too lightly and the sandwich is dry. The whole appeal is that the diner sets that dial. A wedge of roll goes in, comes out heavy and dark with jus, and the rest stays intact for the next bite. That tension, between a dry sandwich and a soaked one, managed in real time at the table, is what the format is for.
The variations keep the jus-on-the-side principle and change what it surrounds. A slice of melted cheese, often Swiss or provolone, turns it richer and changes how the bread holds up to the dip. The Chicago Italian beef is the close cousin that pre-soaks the roll instead of leaving the jus in a cup, trading control for saturation, and Buffalo's beef on weck applies the wet-roast-beef logic to a salt-and-caraway kummelweck roll with horseradish. The pastrami dip runs the same method over cured rather than roast beef. Each of those is its own sandwich with its own discipline and deserves a proper article rather than being crowded in here.