AHRI 640 (I-P/2017):

This standard applies to factory made Commercial and Industrial Humidifiers, as defined in Section 3 of this standard.
Energy Source. This standard applies to electric, gas, or hot water operated Commercial and Industrial Humidifiers including those that utilize steam from a central steam boiler.
Installation. Commercial and Industrial Humidifiers are intended for central air systems or direct in-space applications and may be installed in a mechanical room or outdoors.