In reply to Jan Dembowski.

Actually, Jan, if contained in an appropriate document, “You have permission to use the user name KTS915.” would indeed be a license. It's a license from you. And that would be legally enforceable against you.

I think you are treating the word "license" as meaning only some sort of permission granted by a licensing authority. That sort of license would be enforceable against others, and not just the licensing authority.

The law recognizes both such licenses and, indeed, calls them both licenses. They aren't the same and, as I've indicated, the law doesn't treat them the same. But it does call them both licenses.