Except I wonder are they ? Now there is specialism for the roles of referee and AR I think a good case could be made for saying that the persons with the flags are not referees but following a different craft. If a referee goes down injured it isn't one of the ARs who steps into the role but the 4th Official. So, if anything they are Referee's Assistants but much better to be given their own style, title and dignity of Linos or Linespersons.