Foreign Material (Rule 5-2)
As with the club, the most important question to ask if confronted with a ball which has had something applied to it by the player, or in a post-production treatment, is "Why?" If the purpose of the application or treatment is to influence the movement of the ball it would be prohibited.
Markings applied using a felt-tip pen are not contrary to this Rule. Rules 6-5 and 12-2 encourage the player to put an identification mark on his ball to help ensure that he plays his 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.