Actual cubic feet per minute is the volume of gas and air flowing anywhere in a system independent of its density. If the system were moving air at exactly the "standard" condition, then ACFM would equal Standard cubic feet per minute (SCFM). However, this usually is not the case as the most important change between these two definitions is the pressure. To move air, either a positive pressure or a vacuum must be created. When positive pressure is applied to a standard cubic foot of air or other gas, it gets smaller. When a vacuum is applied to a standard cubic foot of gas, it expands. The volume of gas after it is pressurized or rarefied is referred to as its "actual" volume.