The roast beef po' boy is the one where the meat is sliced and then deliberately undone by its own gravy. A roast is braised low until it is tender, sliced or pulled, and returned to a dark, beef-thick gravy that the kitchen wants soaking into the loaf rather than sitting beside it. The defining quality is that this is a wet, messy, gravy-bound sandwich on purpose: the jus is treated as a primary component, not a sauce on the side, and the loaf is expected to absorb it. It is the meaty, savory counterpart to the fried-seafood po' boys, and its whole character is sliced beef held together by liquid.
The craft is in the braise and the gravy ratio. The beef is cooked until the connective tissue softens and the meat slices tender but still has structure, then the slices go back into a gravy reduced thick with the rendered fat and pan fond so it clings rather than runs off. How much gravy goes into the loaf is the entire calibration: enough to saturate the crumb and bind every bite, not so much that the sandwich becomes a fork plate, which is the territory the debris build occupies. The thin-crusted, glass-crackly New Orleans loaf is chosen because its airy interior drinks the gravy and still, briefly, holds together in the hand. Dressed means shredded lettuce, tomato, pickle, and mayonnaise, set as the cold, acidic counter that keeps a rich, gravy-soaked bite from reading as one heavy note. A po' boy shop builds these to order and hands them over fast, because the structure is already softening when it reaches the paper.
The variations are mostly a question of how far past sliced the meat goes. Push the braise and the gravy and it becomes the debris po' boy, more sauce than solid; pull them back and it is a near-dry roast-beef sandwich. A horseradish or Creole-mustard addition sharpens it. Those readings, along with the rest of the po' boy family of shrimp, oyster, catfish, and cochon de lait, are their own sandwiches and deserve their own articles rather than being crowded in here.