The fried catfish sandwich is defined by cornmeal, not flour. The fillet is dredged in seasoned cornmeal rather than a wet batter or a wheat breading, and that choice changes the whole sandwich: cornmeal fries into a sandy, gritty, almost crunchy crust with a faint corn sweetness that flour cannot give, and that texture against the soft, slightly sweet flesh of the catfish is the identity. A flour-battered catfish fillet is a fish sandwich. A cornmeal-crusted one is this sandwich, and the grit of the coat is the reason it has its own name.
The craft is in the dredge and the fry. Catfish is a soft, fatty, mild fillet, so it has to be cooked hot and fast enough to set that cornmeal shell before the flesh goes from tender to mushy inside it. A buttermilk or hot-sauce soak before the dredge does two jobs: it tames the muddy edge some catfish carries and it gives the cornmeal something to grip so the crust does not slough off in the pan. The fillet lands on a plain soft bun, never a crusty roll, because a hard roll would overwhelm a delicate fish and break the textural argument the cornmeal is making. Tartar sauce or a remoulade supplies the acid and fat the lean fish lacks, applied to the bun rather than over the crust so the coating stays crisp, with flat pickle and sometimes a slice of raw onion as the cold counter. It is a fish-camp and meat-and-three build, fried to order and eaten fast, because cornmeal crust softens quickly once the fish steams under it.
The variations follow the catch and the dredge. A blackened fillet drops the cornmeal for a dry spice crust and a hot skillet, which is a different sandwich. A spicier Delta dredge leans cayenne into the meal; a po' boy build moves the same fillet onto airy Louisiana French bread and dresses it. Each of those is its own regional reading and deserves its own article rather than being crowded in here.