What is Vacuum ( Water Lift)?

Inches of water lift. (Usually given at 0" air opening). Central vacuums don't vacuum water, but the motor's force is measured that way. 
A vacuum's ability to lift is a valid measurement. In fact, it is called vacuum, or water lift. The suction capacity of a vacuum is not the key to effective vacuuming. The real key is how much air the vacuum moves and how fast it moves it (see what is CFM). Water lift becomes somewhat important when the air opening size is smaller - for example in air driven brushes, because the strength of the suction keeps the air turbine spinning when tension increases.
Water lift measurements will fluctuate based on the actual amount of voltage the motor is receiving, the altitude, air temperature, and the barometric pressure.