Cpm is a capability index, also known as the Taguchi capability index, that is a function of the specification limits, mean of the process, and a provided target, T.
Sometimes the customer may specify targeting the upper or lower end of the tolerance. This could be due to past experiences, part weight reduction, and many other reasons.
The value of Cpm is always > 0 and assumes normally distributed process output and uses the sample standard deviation calculation for sigma (within).
The links below explain other process capability indices.
Six Sigma Modules
The following are available
Green Belt Program (1,000+ Slides)
Cost of Quality
Cause & Effect Matrix
Central Limit Theorem
Correlation and Regression
MTBF and MTTR
... and more