The game consists of two parts. First a leg of 501 and then a part with 5 numbers around the board.
For the 501 difficulty level you decide what score you need to reach with a throw, to make it count.
If you for example choose 100, you need to score at least 100 or your score will not be counted in the leg.
Also select if you're only allowed to score on a certain number, or as in normal 501 - any number.
Once you're down to your minimum score, you need to finish in one throw. That means, if your minimum score is 100 and you end up at a score lower than 100 you're busted. You can also select to save score by throwing at bull if you can't reach the minimum score.
If you selected to play more than 5 numbers of the clock section, you're back to a leg of 501 when you've finished the clock section.
Decide if you're going for doubles, trebles, singles or any of them.