(1) Conforming Ball Must Be Played. In making each stroke, a player must use a ball that conforms to the requirements in the Equipment Rules.
A player may get a conforming ball to play from anyone else, including another player on the course.
(2) Deliberately Altered Ball Must Not Be Played. A player must not make a stroke at a ball whose performance characteristics have been deliberately altered, such as by scuffing or heating the ball or by applying any substance (other than in cleaning it).
Markings applied using a felt-tip pen are not contrary to Rule 4.2a(2). Rule 6.3a encourages the player to put an identification mark on his or her ball to help ensure that he or she plays their own ball throughout the round. There are no regulations to limit what or how many markings can be applied to the ball by the player, provided its original markings can be discerned.
Manufacturers may submit balls to The R&A and USGA for testing. Twenty-four samples of each ball type or model must be submitted to obtain a conformance ruling. If the samples conform to the Equipment Rules, the model is included on the List of Conforming Golf Balls, which is updated on a monthly basis. The current list can be found on The R&A’s and USGA’s websites. Full details of the golf ball submission procedure can be obtained from The R&A or USGA.
Balls remain on the List for one year. Random tests are carried out by The R&A and USGA on selected balls to ensure that the ball characteristics remain the same. If a conformance issue is identified during random testing, the ball may be removed from the list.
Normally, only the Committees in charge of competitions involving expert players (professional-level players and/or players at the highest levels of amateur golf) should introduce a Term of Competition requiring a player to use a ball included in the List of Conforming Golf Balls, the penalty for breach of the Term being disqualification. However, if this Term is not in place, then the assumption is that a ball used by a player conforms unless there is strong evidence to the contrary e.g. a player used a 1.62 inch diameter golf ball (a ball which used to be in circulation and was permitted under the Rules before finally being prohibited worldwide in 1990).