I dont think football expects you to caution someone who has a belly button piercing !!
IF, and it's a huge IF, you spot said offending article pre match, based on what I read, as referee, you tell them to remove it or they cant play??
good luck with that.
as ever, if your thing is to caution, or prohibit someone from playing who has a belly button piercing, that's entirely ones prerogative
Am just stating that person wont be me. Which, I don't ask you to agree with but on a forum on which am a contributor, am allowed that stance !
two opponents in game, diff league teams, play for same international team. You have caution belly button offender and she then says "ok but blue 6 (her international team mate) has the same piercing as me ". Whats been the crime of the first offender? Wearing piercing? Or, showing it?
Your role as referee is to be fair and consistent across the board. You cant turn round and say "well your getting a caution because I saw yours even though there are 9 other players on the park wearing the same thing " !!!!!
whole can of worms that can be prevented and handled at source by, not being pedantic and simply get on with the task in hand