How does FRL work? 3

Okay...at long last, I have to ask. What the heck is FRL and how does it work? I understand that its "A number assigned to indicate the Facility Restriction Level" and I'm assuming that means its assigned to things as a way of limiting them from doing things on a 'sliding scale from 0 to 7'

Okay okay, so aside that, is what FRL restricts just arbitrary? Is there a set restriction for FRL and WHAT does it restrict? (seriously, this is some sort of crazy alien numeric identifier for god knows what where I'm concerned. If COS and COR handle most things, FRL makes no sense to me. Yes, okay, so 0 is most restricted, 7 is least restricted. What is it restricting? How does this restriction ease in higher numbers? Etc?)

It's been driving me loopy, and right at the moment I have a phone set with an issue that the phone cannot be FAC-code forwarded, and the only configuration problem I can see is that FRL on the phone doing the forwarding is 0 (most restricted) to two phones with FRLs of 4 (less restricted).

So I need to understand FRL better at this point...