You can find a detailed explaination of the SVT Functions in the PDF user manual.
Online User Manual SVT 4.0:
This is what it lists about the frequency range on THD+N, page 15-6:
Input a frequency range to refine the measurement. The measured power in
the total signal is distributed across the entire measurement bandwidth as a
result of the noise in the measured signal. You can limit the measurement
to a particular frequency range, exclude low frequency noise, or exclude
high frequency noise by selecting an appropriate frequency range. For
example, if you want to specifically exclude DC energy from the
measurement, you should set the start frequency to 8(df). If there are
frequency components of interest below 8(df) that are not DC, reduce df
by increasing the measurement duration.
I hope this helps.
Regards
Karsten