Susan hit her second shot on #1 over the creek but to the right into the long grass. She wasn't sure if she would find her ball so this was a perfect time to play a provisional ball. Susan ended up finding the ball, but it was unplayable, so she had to use the unplayable ball rule. (See Rules Tab above for more details about that rule.) Once she found her ball, the provisional ball may not be used.
What is a provisional ball and when do you play one?
You hit a provisional ball when the ball may be lost outside of a hazard. You also hit a provisional when there is a possibility that the ball may have gone out of bounds. You announce to your playing partners that it is a "provisional ball" (and it should be marked differently from the first one).
If you find the lost ball you must play that ball, even if you don't like where it is, or if it's unplayable.
If you cannot find the lost ball, then your provisional ball becomes the ball in play.
If you don't find the lost ball, and you haven't played a provisional ball, you must actually go back to where you hit your previous shot. That's why we play a provisional - it saves a lot of time. (You are not allowed to just drop a ball outside where you think it may have become lost.)
Note: you may not play a provisional if the ball is likely lost in a hazard. Once you play a ball over the hazard, that becomes the ball in play. So it's a good idea to make sure the ball really is in the hazard before you play the next shot.