The katsu sando is a fried cutlet between two slices of soft bread, and the cutlet is the entire argument. A breaded, deep-fried piece of pork, beef, chicken or seafood is sauced with a thick, sweet, Worcestershire-style tonkatsu sauce and pressed into thick shokupan. The contrast is the point: a hot, crisp, crumbed exterior against cool, pillowy bread, with the sauce soaking just far enough into both to bind them.
The craft is the fry and the timing. The cutlet is breaded in coarse panko and fried so the crust shatters rather than softens, then rested briefly so it stays crisp against the bread instead of steaming it. The sauce is applied with restraint, enough to season and bind, not so much that it turns the crumb to paste; cabbage or mustard is sometimes added for cut. The crust is trimmed so each bite is even, and the sandwich is cut to show the pink or golden interior.
The variations are the cutlet itself: tonkatsu in its loin and tenderloin cuts, premium kurobuta and agu pork, wagyu beef, chicken, the minced menchi and croquette and seafood fries. Each is the same fried-cutlet logic given a different protein, and each deserves its own article rather than being crowded in here.