In the old days, when referees were appointed to league instead of being pooled. Higher Grade referees could only referee on their league or at grassroots level. There is a true story of George Courtney not having a Football League match, so if he wanted a game he did a local leageu match in the Durham area. How about that an FIFA referee doing the Saturday kickers league!
That must have changed a long time ago, I frequently did supply league middles when they were short and I didn't have a contrib or panel game. Or when I was coming back from injury.