One of the characters in a TV show "Seinfeld", season 2, episode 12 (1991) is considering his career options and rejects projectionist and a stable boy as possible choices, because they probably are "a union thing". I understand it means something about labor unions. What exactly makes these jobs inaccessible?
You can't work in a union job unless you are a member of the union.
You cannot join the union until you have proof you have worked in that job.
This used to be often said about the UK actors' union, Equity, until the law was changed to outlaw 'closed-shop'.