A "commitment player" just means that the guy has to play 4 ATP500 tournaments (or Monte-Carlo) in the calendar year in order to avoid a zeropointer in his rankings, AND in order to share the good $$$$$ from the bonus pool.
That's the only thing it means.
As for other top-50 players, usually they have enough with ATP world tour tournaments, sometimes they ask a wild-card in a challenger in their own country (Nieminen in Helsinki in a few weeks for instance, he plays that challenger nearly every year because that's the biggest tournament in his country) and/or if they get good money.
Don't you also mean 250's? What is the bonus pool?
Oh yeah, right, forgot about those.