I guess what's not clear to me is...where do you draw the line between a close friend, and someone who's closer?
Is there a line at all there? Or is it something that can only be self-described, in the eye of the beholder?
Put a different way,
What is the difference between a polyamorous person's NS partner, and a mongamous person's close friend?
Are there genuine differences here? Does it just come down to how you personally feel about the person?