That's it. Your awesome Thank you
You're welcome.
To address more specifically what it's used for (now that I'm more fully awake): This menu is used to bias the White Balance preset(s) in whatever way you wish. White Balance, to oversimplify things a little for the sake of brevity, is the color of the ambient light. So, for example, lets say you're using the "Auto" White Balance setting but in your personal opinion skin tones when using this particular setting consistently look washed out, or your landscapes taken at Golden Hour are looking a little too blue. Well, you can use this menu to bias the "Auto" setting towards Magenta or Yellow to counter-balance what you're seeing. I mention those scenarios purely as possible practical examples; I'm definitely NOT suggesting anyone actually needs to DO anything in this menu, but you can if you so choose. Just remember it adjusts the default WB for the
entire image because, as I said, White Balance is (essentially) the color temperature of the ambient light so while the skin tones, specifically mentioned in my example would be "corrected", the whole image will be affected. Hope that makes sense. And since the menu doesn't make it clear: A=Amber, B=Blue, G=Green and M=Magenta.
Some people will tell you Canon cameras tend to have a more pleasing, warmer overall White Balance compared to Nikon bodies, or conversely that Nikon camera bodies have a more "accurate", if also somewhat sterile or "cooler" overall WB. I don't want to enter into that particular debate but if you wanted to simulate the Canon WB in your Nikon body (as some reportedly do) you could bias one of these settings toward Amber or Orange or Red or some combination thereof, using this menu. That's just another practical example of how this menu could be used.
If you do decide to experiment with these settings remember you've made change because nothing on the camera will remind you of them (not like, say using Exposure Compensation) and any changes made will "stick" until you go back into the menu and change them again. For those who know A) exactly what they're doing and B) exactly what they want out of their WB setting(s), this menu is extraordinarily powerful and helpful. It's also completely reversible should you decide you want to experiment.