The Southern biscuit sandwich is defined by the carrier, not the filling. A tall, flaky buttermilk biscuit, split warm, is the entire reason the sandwich exists, and its tender, fatty, faintly tangy crumb is a deliberate counter to whatever salty thing goes inside it. Bacon, sausage, ham, egg, and cheese are all interchangeable here; the biscuit is the fixed point. Get the biscuit right and the sandwich works with almost any filling; get it wrong and no filling rescues it.
The craft is in the biscuit and the heat. A proper buttermilk biscuit is built on cold fat cut into flour so it stays in sheets, which steam apart in the oven into layers rather than a uniform crumb, giving a structure that is rich and short but still strong enough to hold a hot filling without crushing to paste. It has to be split while warm and filled fast, because a biscuit's window is narrow: too cool and the crumb tightens and goes dry against the egg. The filling is matched to that richness rather than competing with it. A salt-cured country ham is so assertive it needs the biscuit's blandness as relief; a sausage patty is griddled to a hard edge for contrast against the soft crumb; the egg is kept loose so it acts as a binder, and a slice of cheese melted against it glues the build. A smear of sawmill gravy or a swipe of jam or honey is the regional accent, sharpening or sweetening a structure that is fundamentally soft and fatty all the way through.
The variations are codified by the filling and the part of the South defending it. The country ham biscuit runs salt and funk against the bland crumb; the sausage biscuit and the fried chicken biscuit lean on a hard-fried interior for contrast; the sausage-gravy build floods the biscuit until it is a knife-and-fork plate; the honey-and-butter version pushes it toward the sweet end of breakfast. These belong to the wider American breakfast sandwich family, where the carrier is matched to the region, and each deserves its own article rather than being crowded in here.