What is a provisional ball and when should I play it? See Rule 27
A provisional ball is a ball that is played when the ball that you hit may be lost (other than in a water hazard) or out of bounds. It is a procedure used to save the time as the relief from a lost ball is to replay another ball from the place where the original ball was hit (stroke and distance).
Play a provisional ball when your ball has gone into the woods or tall rough around the fairways or when you are not sure that you will be able to find your ball. Do not play a provisional ball if you are not sure whether your ball has landed in a water hazard. A provisional ball is not just used on your drive, but should be used any time you think that you may not be able to find your ball through the green.
You do this by announcing to your group that you are going to play a provisional ball, and describe the ball (I’m playing a Titleist #3) that you are about to hit.
Then when you go forward to search, your fellow competitors will know which ball is your first ball and which is your second ball.
If you do not announce that it is your provisional ball, it becomes the ball in play and a 1 stroke penalty is incurred at that time.
If the provisional ball does not go as far as your original ball, you may play another stroke(s) until it reaches the area where your original ball is believed to be. If you find your original ball, the provisional ball is abandoned and none of the strokes with the provisional ball count in your score. If you do not find your original ball, the strokes played with the provisional ball count plus the 1 stroke penalty (stroke and distance). If you make a stroke with your provisional ball from the place where your original ball is likely to be, OR from a point nearer the hole than that place, then your original ball is considered lost (even if found) and the provisional ball comes into play (again under stroke and distance Rule 27.1(A).