The capacity on an OEM tag is the rated capacity of the valve on the media on which the valve is set.  All valves intended for liquid service will be set on water and will have a nameplate capacity of GPM of water.  All valves intended for gas/vapor service will be set on air and will have a nameplate capacity of SCFM air.  All valves intended for steam service will be set on steam and will have a nameplate capacity of PPH steam.

Depending on the application of the valve, the capacity could be determined at differing overpressure percentages.  For instance, Section I steam valves would be determined at  3% overpressure, Section VIII code certified valves would be determined at 10% overpressure, and non-code liquid valves could be determined at 25% overpressure.