What parameter evaluates distortion of an AFSK signal caused by excessive input audio levels?
Why When audio tones are fed too hot into an SSB transmitter, the transmitter is driven out of its linear range and the tones mix with each other, producing extra unwanted sum and difference products. That is intermodulation distortion, and software such as PSK31 monitors reports it in dB below the desired signal (roughly -30 dB or better is considered clean). Backing off the audio drive or ALC action reduces IMD and keeps the signal narrow.
Watch out Signal-to-noise ratio measures how far a signal stands above the noise floor at the receiver, not how badly the transmitter is being overdriven; it can look fine while the signal is badly splattering.
Too much audio equals too much IMD. Watch the IMD number in dB, aim for -30 dB or better.