Blending Question

[JFASI]

I have a question that's been bugging me for a while (not unlike most of my questions). When a render mode, say, XOR, does its math, like exclusive or in XOR's case, what is it comparing? Better put, does it compare the colors of two pixels, or the brightness, or some other more transient quality? i.e. Maximum blending, does it display the brighter image, more colorful, etc.