Tying “fork” to “repositories” is nonsense, because software forks have existed longer thsn e.g. git.
How do you define “repository”, such that it makes sense? Is it only Git repositories? Any version control system? How about a .zip-file placed on an FTP server?
The point is, it’s not at all clear, because Github has it’s own definition of what “fork” means. I’m honestly not sure why it’s so hard to grasp.