[MM3-users] List nonmembers

Under Users for a list, there is a category for List nonmembers. What does this term mean? [It's rather curious philosophically when you think about it, because I would have thought that the category [all humans - list members] fits that category term.]

But seriously, I don't understand it. I have several email addresses there and I don't know what do with them, if anything.

Andrew