About distortion
 
Harmonic Distortion

Intermodulation Distortion - IMD
measured with 60 Hz and 1 kHz in proportion 4:1


Differential Frequency Distortion - DFD
measured with 9 kHz and 10 kHz in proportion 1:1

Figures above borrowed without permission from Rohde & Schwarz UPL manual
(But I don't think they will complain)