This is a single point on a curve, but it helps to understand the whole curve. Our P1dB utility plots this curve at a given frequency by sweeping the input power to the device:

If at the same point, or any other amplitude, the phase of the signal shifts, those constellation points will be affected. Generally again, the high-power ones will move around the circumference of a circle. When this phase shift is amplitude- (vector length) related we have phase modulation due to amplitude modulation or AM to PM conversion. The outer constellation points move around a circle, the inner ones less so and the constellation becomes twisted.

Clearly these amplifier distortions are important, and can be critical in any modulated RF system more advanced than Morse Code - even FM to a degree. If a data code is too far misplaced, it cannot be recognized and corruption occurs. Once an amplifier or any potentially nonlinear element has been characterized for these distortions, it will either be used within its limitations or, much more commonly now, the input will be predistorted to compensate the device. Why? To save battery life! We need to drive low-power amplifiers as far into compression as we possibly can. It is hard to imagine just how much work has been done in this area!

One further point. A great many comms signals hop around the carrier frequency, and the distortions in a device vary with frequency. That is why our P1dB utility can perform the measurement at up to 201 different frequencies in a sweep of frequency and power.