The phrase describes a heater system designed to cover a broader range of infrared wavelengths. It is useful, but it should never be the only reason you choose a sauna.
MAAX positions PRO 5 around far- and mid-infrared heat in a compact Canadian Hemlock cabin.
PRO 10 adds full-spectrum infrared elements alongside carbon heaters in a larger Canadian Red Cedar cabin.
Heater placement, bench position, electrical requirements, materials, controls, warranty and how the sauna feels matter more than stacking up marketing buzzwords.