To demonstrate the "Track first harmonic" option in Exact frequency mathematics we have made an example. 

We have generated the signal with 3 sine waves. The result of that signal is shown on the FFT.

After the signal was generated, we have created two Exact frequency mathematics, one with and one without the "Track first harmonic" option enabled. 

At the beginning, both the mathematics show the same 100Hz, which is the highest peak in the frequency domain.

After we change the amplitude of the sine with 200Hz to the highest the mathematics show different results. Exact freqency mathematics with "Track first harmonic" option enabled will show 100Hz while the one with this option disabled will show 200Hz.