Two mounted umpires , one refree in stands and 2 goal judges together control the conduct of a match. Goal judges  help umpires when they are asked however the decision of umpire is final. 

Genrally, the polo ground is diagonally divided in two halves. One back line for one umpire each . Out of the 2 umpires , one genrally follows the line and second moves parallel to the game to judge distance between players. Both these aspects ie line of ball and distance between involved players have a role to play in deciding the validity of a move. Its easy to see the logical trail , which makes the umpire with farther backline monitor distance and the one with nearer back line monitor the line of ball.

Refree sits in the stand and observes the game. Doesn’t interfere normally unless both umpires disagree with each other and come to him for his verdict. Once referred, the decision refree gives is final.