Flow in m3/h of water at +5 to +30°C passing through a device (valve) in a given direction based on a pressure drop of 1 bar and an upstream pressure of 6 bar.
The practical benefits of the Kv value (of valves) are disputed, partly because it equates the actual flow resistance of compressed air with that of water. It also ignores the fact that resistance in both flow directions associated with a 4-way valve can vary greatly (by up to 50%).