Measuring the 1dB compression point is a lot of work as you manually have to step through many signal levels to find the level where the detected signal is 1dB below the expected level.
Using the internal step attenuator and the LINEARITY measurement in the MEASURE menu most of the manual step are automated.
The green line in the picture below shows 9 measurements shown for each 1 dB step starting at 29dB attenuation.
Using an external signal generator at any of the supported low input frequencies at a level above the expected compression point level (such as?+10dBm) will allow a quick measurement of the linearity.
As the green line shows little disturbances below where the compression starts it is safe to conclude the internal step attenuator is very linear.
