Manchester United midfielder Casemiro has a catalog of dossiers on referees around the world.
Casemiro's dossiers include where a referee is from, if they have a job outside football, how strict they are and what their style of refereeing is like.
The Brazilian international's friend and former teammate Bruno dos Anjos has revealed that he meticulously plans for matches including having documents that analyse referees.
Dos Anjos told the Telegraph: "He plays in a role where you talk a lot to the referees and are more likely to receive a card.
"So it's important that they understand what he says and that he's able to argue with them if necessary as well.
"It may be helpful in certain situations."