i know on the ITF circuit you have to fill-in forms and state WHY you are applying for one.
Sampras always asks for a WC into Queens cos he can't be bothered to enter , and asks one to be held back for him.
depends on your ranking, but certain BIG name players are likely to get one if a) they don't enter and b) too low to get in.
directors often save a few of these. if not then theu given out to local players.