A single-zone mini-split pairs one outdoor condenser with one indoor head — the simplest, most efficient ductless setup. A multi-zone system runs several indoor heads (the Department of Energy notes many models support up to four, and some lines go to five or more) from a single outdoor unit on one refrigerant circuit, so each room gets its own thermostat without a condenser per room.
The trade-off is efficiency and control. Single-zone units earn the highest SEER2 and HSPF2 ratings on the market because the compressor is matched to exactly one load. Multi-zone systems generally rate lower, and they carry a subtler risk: the smallest head sets a minimum capacity the shared compressor can't easily modulate below. When only one small room is calling on a mild day, an oversized multi-zone can short-cycle — switching on and off instead of running low and steady — which wastes energy and undercuts the humidity control that makes ductless comfortable.
The practical guidance: size each zone honestly with a load calculation rather than defaulting to the biggest head that fits, and don't add zones 'just in case.' For a single room or an addition, single-zone is usually both cheaper and more efficient. Multi-zone earns its premium when you genuinely need independent control of several spaces and running separate single-zone systems would mean too many outdoor units. A good installer will show the math, not just the head count.