What does the prefix "x" mean in the site address?

Question

Web development

For the first time I encountered the format https://x.facebook.com/ - what is it?

I know there is https://m.facebook.com/ - it's the same content as on the desktop version, just for mobile devices.

What does "x" mean?

Answer 1

It's a subdomain, not a prefix.
Or a third level domain.
It does not mean anything, just like m.

Answer 2

A simple site mirror.

Answer 3

Yes, anything, as Facebook progers came up with, can designate a group of file servers or cdn or something else that gives content