I just need to know the purpose or the duty of the centre line judge in international matches. I mean the middle one who sits opposite to the face of the net.
I'm not sure which one you mean, so let's explain both options: The service judge (blue box) assists the umpire by calling out service faults, which can be hard to see from high up in the umpire's chair. Additionally, the service judge takes on some management functions, such as giving out new shuttles and store broken ones, put on a marker during intervals or after the game. Players are expected to shake hands both with the umpire and service judge after the game. A good service judge also assists the umpire discreetly. Service judges are generally umpires themselves; tournaments usually rotate the umpires between service judge and umpire duty. Service judges wear umpire's clothes (i.e. black) and cannot be overruled by the umpire. In some instances, a service judge may also function as a line judge for the line they sit on, although only when they are less than two line judges. The line judge (red circle) indicates whether a shuttle goes in or out. The center line judge (the one marked with the circle) is only needed during the service, because the service must go diagonally. This position is informally known as one of the sleep lines, since it is very rarely called upon - in high levels of play, it's quite rare for a shuttle to go to the opposite half court instead of diagonally, and in any case, this line judge can safely take a nap after the receiver has hit the shuttle, and only needs to wake up at the start of the next rally. Line judges usually wear a tournament-specific uniform, if any. In theory, they don't need to be umpires, although they often are (usually with a lower qualification than the tournament requires). They can be overruled by the umpire (or a line call challenge) if necessary, for example if they have fallen asleep.
