Also, in the advanced settings, this can be done by "group" or "each". If you set a player to 50% exposure:
Group means, it knows you're going to try to make 10 lineups, it will keep the player in the pool until he's in 5 lineups.
Each means, it knows you want him in 50% of your lineups, he will be excluded in every other lineup.
Regardless of setting, this gets REALLY funky when you adjust the exposure settings of too many players at once, and you could possibly end up with junk because it will decide to remove all of your good players simultaneously for some lineups.