Bǐng (饼) is the general term for flatbread across the Chinese kitchen, not a single sandwich but the umbrella under which a whole family of round wheat breads lives. It belongs here as a category marker, because so many of the country's handheld and stuffed forms are a bǐng doing the structural work: holding a filling, carrying a sauce, or standing in for a slice. The defining idea is breadth on purpose. The word covers anything flat and round made mostly from wheat, which is exactly why naming it tells you the format and almost nothing about the specifics.
The craft is whatever the particular bǐng demands, and the variety is the point. Some are thin and supple, rolled almost translucent and cooked briefly on a dry griddle so they stay foldable, the kind that wrap roast duck or shredded fillings. Some are thick and dense, laminated with oil into internal layers and griddle-then-baked firm, the kind split and packed with braised meat. Some are leavened and soft, some flaky from folded fat, some studded with sesame and baked against a hot oven wall, some shallow-fried until blistered and crisp. Good execution is judged against the job: a wrapper bǐng should be pliable and fine-textured so it rolls without cracking, a pocket bǐng should have a firm shell and a layered tender interior so it holds a wet filling, a flaky bǐng should shatter cleanly rather than chew like raw dough. The shared failure modes follow from misjudging that job: too thick and doughy where thin and supple was needed, too hard and biscuit-like where soft was wanted, or under-cooked so the center stays gummy whatever the style.
From there it shifts into named members, each of which earns its own treatment. The sesame-topped baked flatbread, the thin duck wrapper, the layered scallion pancake, the thick sugar-cored griddle bread, and the pan-fried stuffed pocket are all a bǐng in the broad sense but distinct preparations with their own methods and ratios. What "bǐng" tells you, on a menu without further qualification, is only that the thing in question is a flat round wheat bread of some kind. The specifics, leavened or not, thin or thick, plain or stuffed, sweet or savory, are what give each its identity, and those deserve their own articles rather than being crowded in here.