Distorsions...

Hello. Please, why do i have such distorsions at 12K6 hz?

Probably because the reference is the harmonic frequency. The 2nd harmonic reference is at 25.2 kHz which is likely after your measurement rolls off so the fundamental there is very low. You can choose whether or not to use the harmonic frequency as the reference in the graph controls, the help explains why that may be helpful.

That is true. You are the best.

Please could you tell me when we must use "Plot harmonics at the harmonics frequency"?

Plot harmonics at the harmonic frequency changes the way the harmonic orders for sweep measurement distortion are plotted - rather than plotting the distortion at the frequency of the fundamental it is plotted at the frequency of the harmonic, so (for example) the 2nd order harmonic distortion level for 1 kHz would be plotted at 2 kHz, where the distortion occurs. This makes it easier to correlate harmonic levels with the level of the fundamental at the harmonic frequency and also helps distinguish the effects of external noise, since noise affects all harmonics at the frequency at which it occurs. The THD trace is not affected by this control, it will continue to show the THD value corresponding to the harmonic levels at their fundamental frequency. However, when plotting harmonics at the harmonic frequency the level of the fundamental at the harmonic frequency (which in this graph mode is the cursor frequency) is used as the reference, so Use harmonic frequency as ref will be selected and the control for it disabled. If that was not previously selected the THD trace will update.