All defined terms are in italics and are listed alphabetically in the Definitions section - see pages 2235.
To determine if it is unfit for play (Rule 5-3);
For identification (Rule 12-2), in which case it may be cleaned only to the extent necessary for identification; or
Because it is assisting or interfering with play (Rule 22).
If a player cleans his ball during play of a hole except as provided in this Rule, he incurs a penalty of one stroke and the ball, if lifted, must be replaced.