I know Kate already gave this answer, but just the same:

My educated guess would be that this does not mean there is a physical column of liquid inside a cilinder, the text just refers to the fact that the pressure in the tank corresponds to 7 metres of fluid column (in the case of water: app. 0.7 bar); the use of these 'units' could be related to the fact that the maximum *rise height* of certain pumps is app. the same for fluids of different specific gravity, thus eliminating the need for conversions