"Racketing Up" for a court

Question

i play regularly one popular public tennis courts. There is a posted sign instructing would-be players to racket up for courts when the courts are busy. Exactly how does racketing-up work?

There are 5 courts, and 5 matching racket holders each with an even and odd marker. What does this all mean?

Answer 1

it means you have to racket up

Answer 2

It means when courts are not available, you mark which court you want to line up to play next.  If there is a holder, put your racket there to reserve the court when it's available :)