Traditional surround sound (5.1, 7.1) mixes audio into a fixed number of channels. Atmos instead treats sounds as objects with positions in 3D space; the processor renders those objects to whatever speakers you actually have — including height channels, either in-ceiling or up-firing modules that bounce off the ceiling.
The notation changes accordingly: a 5.1.4 system means five ear-level speakers, one subwoofer, and four height speakers. Done properly, Atmos is genuinely immersive — rain overhead, a helicopter passing across the ceiling. Done lazily (two up-firing modules in a reflective room) the effect is subtle. Ceiling speakers, correctly placed, are the gold standard.