> *Which cross-over frequency is this referring to with respect to the
> DC-DC Converter Switching frequency*

It's more than likely referring to the the LC resonant frequency of the energy storage components within the DC-to-DC converter. It likely refers to these because below the "so-called" cross-over frequency, the LC circuit does not introduce significant phase shift within the loop (a good thing). 

However, above the cross-over frequency there is virtually a total phase inversion due to the LC changing its characteristic rapidly. Consider 10 uH and 10 uF for the energy transfer components and look at the green (phase) response below: -

[![enter image description here][1]][1]

[Link to Interactive calculator](http://www.stades.co.uk/RLC%20filters/RLC%20LPF.html#RLC_low_pass_filter_type_2)

Slightly below 15.9 kHz, the phase shift is quite close to 0° and this poses no threat of causing loop instability. However, slightly above 15.9 kHz, the phase has shifted nearly 180° and this can really "shake the ground" when it comes to stability. This is why compensation circuits are added within the control loop to advance the phase shift (a counter measure) and prevent the 180° oscillatory condition arising.

So, as a "guide", you keep the resonant frequency of the energy transfer components significantly below the switching frequency. But, you also need to keep that resonant frequency high to avoid the 180° phase shift occurring at too low a frequency; you don't want it to occur at a frequency where the "compensation" isn't active. 

You also want to keep the resonant frequency high so that your closed-loop control system can react quickly to load and supply changes. But there's another criteria to consider and that is the output ripple voltage. 

You need to keep the resonant frequency a decent way below the switching frequency so that switching artefacts are kept to an acceptably low level on the output waveform. The LC is a great low pass filter for this and in the above picture, you can probably see that if the switching frequency were 159 kHz, the attenuation of the switching voltage will be 40 dB. So, if the switching is 10 volts p-p, the resulting 1st harmonic on the output waveform will be 100 times lower at 100 mV p-p.

Why 8:1? Why not 10:1? The fact is, that it's a rule of thumb and like most rules of thumb, you can choose to push the rule this way or that way depending on your most dominant needs. 

Hopefully, the information above will allow you to see that the choice of LC cross-over frequency is a compromise based on juggling these somewhat opposing constraints: -

 - The loop response to load and supply voltage changes (Fc high)
 - Ensuring the compensation circuit is effective at the resonance (Fc "right")
 - Minimizing output ripple voltage (Fc low)

  [1]: https://i.sstatic.net/TjSeI.png