It's a system role introduced in 2.x (I think It turned up when we went from 1.9 to 2.2 but I'm not sure) that allows people assigned to it access to any course without needing to be enrolled but gives little, if any, permissions to edit.
It definately isn't a custom role - see screen shot. Also I'm the only admin here who handles this area of Moodle. This role appeared in a 2.x upgrade and I remember looking forward to it because it solved a problem one of my teachers had at the time.
I've just checked on my test server and that role is not listed under Assign roles in System. I can only assume from that, that it was a feature that was introduced and quickly removed. I wonder why we still have it.
As it's no longer supported and the incremental upgrade we've done recently has broken the way the permissions work on it, I guess my only real option is to make a custom role that fulfills the same criteria.