Falafel b'Khubz names the falafel sandwich by its bread: khubz is the thin, soft Arabic flatbread, and the b' simply means "in." It is the everyday default form of falafel across Lebanon, the one most people picture, and the angle is how little the bread asks of itself. Khubz is pliable, almost neutral, and thin enough to disappear into the bite, so it frames the fritter and the sauce without competing, which makes the fry and the pickles carry the sandwich entirely.
The build is the standard Lebanese falafel assembly wrapped in soft flatbread. The fritters are ground from soaked chickpeas, often with fava, blended with parsley, cilantro, onion, garlic, cumin, and coriander, shaped small and deep-fried to order so the shell crackles and the inside stays green and tender. They are laid along a sheet of khubz, dressed with tahini sauce thinned with lemon and water, then tomato, cucumber, parsley or mint, onion, and the pink pickled turnips that ride alongside so much Lebanese street food, sometimes a chili paste for heat. The bread is rolled tight into a cylinder so the whole thing holds in one hand. Good execution fries the falafel fresh so it stays crisp inside the wrap, keeps the tahini tangy and pourable rather than pasty, and rolls the bread firmly enough that nothing slides out the end. Sloppy execution wraps soft, pre-fried fritters that go greasy in the bread, over-sauces until the khubz turns to mush, or rolls it loose so the filling escapes after two bites.
It varies mostly by the fry and the sauce, since the bread is the constant that defines it. A shop that fries to order and pours tahini with restraint produces a clean, balanced wrap; one that batches both produces a heavy, sodden one. The directly related forms swap the bread out: the same filling stuffed into a pocket pita rather than rolled in flatbread, or rolled in thin griddled saj as falafel b'saj, which eats drier and more delicate. Eaten in the morning the same wrap doubles as breakfast. What holds steady is the point of the name: this is falafel in soft Arabic bread, where the bread's job is to vanish and let the crisp fritter, the sour turnip, and the nutty tahini do the work.