On the back of their jerseys, football refs and umps have letters. R/ F etc. What do the refer too?

It refers to their positions:

R= Referee

HL= Head Linesman

LJ= Line judge

U= Umpire

BJ= Back Judge

FJ= Field Judge

SJ= Side Judge

the letters represent their posision
r=referee
u=umpire
bj=back judge
fj=field judge
etc.

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