The USPA rules are available for ~$10 from the United States Polo Association, 4059 Iron Works Pike, Lexington, KY 40511, vocie:(606) 255-0593, (800) 232-USPA, fax:(606) 231-9738 The rules are updated every year, every player should have and study the rules. Spectators would enjoy the game more if they too would know the rules.
An outdoor polo game is officiated by two mounted umpires and a sidelines referee, or three mounted umpires. Three officials allow for multiple points of view and for tie breaking if a call is contested among the umpires.
The arena rules also suggests two mounted umpires and a sidelines referee, the field size typically does not allow three mounted umpires. The arena game allows a single umpire with no referee when three officials are not available.
The rules of polo are loose enough to allow a very broad spectrum of style and tactics. Most of the rules are just common sense and at first simple to describe hence the beginner relies on the "hearsay rule book". This is easy and works until one finds oneself being penalized more and more in tournaments, especially when away from the home club. The problem is a sloppy second hand understanding of the rules. Solution: know the rules first hand, and, almost as important, what is not in the rules.
The USPA rules form the basis for nearly every game played in the USA. Each club makes slight variations, e.g. when a field requires a particular interpetation such as distances for penalty shots. There are distinct rules for both arena and outdoor games. Fundamentaly the rules are the same, differences exist in the interpretation of board plays for arena, also in the penaltiy shots. Even if your games are solely arena or outdoor knowledge of the other rules provides valulable insight.
World wide there are the Hurlingham Rules which dominate the world outside the USA.