Single zone is a swimming zone with combined rest area in the same body of water – cheaper to run, easier to maintain (only one set of filters etc) but you cannot have your seating / rest area at a different temperature as you can with dual zone. You can of course combine a single zone swimspa with a separate hot tub.
Dual zone swim spa has a completely separate hot tub area which has separate heating controls etc and you can easily move between the swim zone and the seating / relaxing hot tub area.