How is the frequency vector being derived?
linspace(0, 1, fix(numel(FTSignal)/2)+1) $\tag{1}$


"$y =$ linspace(x1,x2,n) generates n points. The spacing between the points is (x2-x1)/(n-1)."

So in the above statement it is generating fix(numel(FTSignal)/2) points between 0 and 1 that are equally spaced


  Frv = linspace(0, 1, fix(numel(FTSignal)/2)+1)*Fn;


This generates Frv with just equation (1) scaled/multiplied by Fn

Edited: the FFT signal can usually be represented by half of the total FFT coefficients, since FFT of a real signal is symmetric around mid point, it is uniquely represented by half the coefficients. Adding 1 is for the DC frequency. Take an example an FFT of length 16, is represented by 8+1 coefficients. The "fix" function is present to handle FFT of odd length signal, so that they can be rounded to an integer value. Since these FFT values have been generated under the normalized scale of 0 to 1, it is then normalized by Fn, which can be $\pi$ or $F_s/2$, where $F_s$ is sampling frequency