Maximum signal lever refers to the largest CW or pulse RF signal that can be safely applied to an amplifier's input. Reverse gain is the ratio of power measured at the input of an amplifier to the applied power at the output of an amplifier, also known as isolation. Sigma is the statistical term for the Standard Deviation of a distribution. All things differ one from another to a greater or lesser degree. The measure of how much a distribution varies around the average or middle value is a measure of its standard deviation.
Most distributions take a bell-shaped curve appearance, called a normal distribution.
Sigma performance, SP, tells how far the nearest spec limit is from the average, compared with the sigma value. Thus, 5P equals 3 times the CpK value. The relationship between the process standard deviation and the width of the specification reflects the sigma. Stability of an amplifier is an indication of its tendency to oscillate or generate a signal at its output without an applied input. Hence they are of great importance in system design.