This figure is the commonly used unit to make the power of vacuum pumps comparable.
The unit m³/h refers to the volume of air, which a vacuum pump could suck in one hour in physical standard state (0°C and 1.013 bar).
Because of the better practical relevance, a technical standard state of 20°C is frequently defined. Hence the vacuum pumps of various manufacturers isn't always comparable like that!
Some manufacturers specify different volume flows depending on the type. Like that e.g. higher reference values are being listed with water ring pump.
Usually, vacuum pumps should not generate an output flow, but maintain a vacuum. Therefore, it is a unit, which was only introduced for comparing purposes and does not show the real situation.
In this case, woodworking is an exception. Here, vacuum pumps are primarily used to clamp often porous workpieces on CNC-machining centres. Here, the pumping speed of a vacuum pump is more important than the reachable hypotension. It is not the goal to create a technical vacuum in a closed vessel, but to clamp workpieces onto a surface. Thus, there is also no completely dense system, i.e. the vacuum pump has to maintain a constant output flow rate.