if you have the front sight post, a perfect co-witness, will set the reticle on the tip of the sight post. Some people like it floating above the sight post, for a better field of view on target.
irons visible in the lower 1/3 of the red dot. Basically imagine the top of your FSB in the lower 1/3 of your optic.
allows for a clearer FOV with the dot but still have the ability to use your BUIS without having to remove the optic/mount combo
although we are talking about Eotechs, here is my Aimpoint with a 1/3rd co-witness which I prefer than a perfect co-witness.