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.
Z Score also known as Standard Score, is the method of calculating how many standard deviations in a data set is above or below the mean. The distance between the mean and a given Z score in every normal distribution cuts off a fixed ratio of the total area below the curve. Z scores are particularly informative when the distribution to which they refer is normal.