The dust loosened by the filters' air jets collects at the bottom of the machine. You can empty some machines by opening a valve and letting the dust fall into a barrel or bag. Other units have augurs and vacuum lines that remove the dust. Very minute dust particles which are not filtered and captured in containers are exhausted into the air. Most dust collectors designed for blasting enclosures are rated at 20,000 to 30,000 cfm.
Most contractors understand cfm.
However, they often fail to grasp the importance of inches of static pressure, which refers to sucking power. Sucking power is roughly a function of horsepower, meaning that more horsepower delivers more static pressure. How much you need depends on variables such as inertia, changes in an enclosure's direction, the total volume of the space, and friction caused by obstructions. It's harder to move dust-laden air when there are steel beams in the way, for example. Two machines may have the same cfm rating, but one machine may have two to three times more horsepower and quite a few more inches of static pressure.