I don't think you need to know this. I haven't seen anything about that in kaplan review, although there might be another prep book that deems it important.
Can I take a stab at what it is? Endo and exo, like endocrine and exocrine? Meaning, one refers to molecules (toxins) in the bloodstream and intersitial fluid while the other refers to molecules secreted by glands? This is just a guess...