When a compressor pumps one CFM (cubic foot per minute), that means the intake port inhales one cubic foot of free air (air at atmospheric pressure, which is 0 psig) per minute. (Note: A CFM does not mean in any sense the compressed volume.) So the unit really measures the mass of air flowing per minute, not volume per minute, since a cubic foot of free air is a unit of mass.
This second scale has a mean and standard deviation that have been arbitrarily set. A z score, for example, is a raw score that has been transformed to a scale with a mean of 0 and a standard deviation of 1.
The Z score is just a quantity that you compute by scaling your sample. It is also called a standard score. It is defined via the following formula: [math]Z(X):=\frac{X-E[X]}{sd(X)}[/math], where: [math]E[X][/math] is the expected value of the random variable [math]X[/math].