What's in a Nod

So I've noticed that there are 2 nods guys use to acknowledge one another. The more formal one -- the one I use for a guest walking through the lobby at the hotel -- is a slight tilt downward. It's classy and always returned the same way. Notice I only perform this gesture for the guys -- I reserve a bright smile or a wink for the ladies! The second and more informal one is the tilt upward (this movement can be either slight or exaggerated). I typically use this nod in casual settings.