Tektronix function generator THD
The THD is listed for frequencies up to 20 kHz, whereas the harmonic distortion is given up to higher frequencies. I suspect this is for audio vs RF/analog testing. In the former, people tend to talk about THD, in the case of the latter, they tend to talk about the level of the harmonic with respect to the carried in dBc. 

The RMS value of a sine wave is just the peak divided by \$\sqrt{2}\$, so the ratio between the peaks is identical to the ratio between the RMS values.