Pizza bianca farcita is the general form of the Roman split flatbread sandwich, the bakery's open template before any one filling claims it. What defines it is the bread itself acting as both vessel and seasoning. Pizza bianca is a long flat sheet baked on the oven floor, dimpled, blistered, brushed with olive oil and strewn with coarse salt, chewy inside and crisp-skinned. Split through the side, it becomes a pocket that already tastes of oil and salt before anything goes in, which means whatever fills it is read against a savoury, fatty base rather than a neutral one. The filling and the bread need each other in a specific way: the bread supplies salt, fat, and structure, and the filling supplies the cold, the moisture, or the sharpness the warm flatbread lacks on its own.
Building it well is mostly about respecting what the bread already contributes. The pizza bianca is split while warm and pliable into a wide, even pocket so the contents lie flat and the skin stays crisp at the edges. Fillings are kept loose and layered in waves rather than packed in dense slabs, so the bread can close without compressing and the oil already in the crumb is spread thin across the bite instead of pooling. Because the bread carries its own salt, fillings are chosen and dosed with that in mind: a cured meat is sliced thin, a soft cheese is spread rather than slabbed, a vegetable is dried so it does not weep into the crumb. A careless version overstuffs cold dense bread until it turns to grease at one spot; a good one is warm, restrained, and built so the seasoning of the bread and the seasoning of the filling add up rather than fight.
The close cousins are simply this template with a filling named, and each is its own subject rather than a crowd here. There is the pizza bianca con mortadella, the soft sausage build that leans on warmth, the con prosciutto with its salt-cured ham, the version filled with grilled vegetables and a smear of soft cheese, and the sweet reading with chocolate spread that turns the same bread in another direction entirely. Each is the same oiled-flatbread-as-base idea with one element changed, and each deserves its own article rather than being crowded in here.