The standard HumidX pucks are ideal for most climates and provide a low level of humidity to your therapy, whereas HumidX Plus pucks are designed to provide a greater level of humidification and are much more effective in arid climates/environments with lower ambient humidity. HumidX and HumidX Plus both work with the AirFit N20 and P10, but there is a specific HumidX for AirFit F20 owners as well.