This setting allows you to calculate harmonic distortion as function of frequency (if your measurement is an exponential sweep as will be explained below).
If is checked, harmonic distortion will be computed.
You can select either Separate or Power Sum as shown below
If Separate is selected, you can calculate and plot individual harmonic distortion components.
If Power Sum is selected, the selected harmonics are summed in one curve. Click the button to open the Harmonic Distortion Settings shown below.