In the theatre industry the union's work to limit supply and demand is usually along the lines of assigning work by seniority and seniority based on how much work someone has done, so when you first get into IATSE, you have to be really flexible and available for whatever work comes along so that you can accumulate the time to get more work.

This somewhat limits entrants. Some IATSE locals use different variations on that work assignment strategy and some entrants get work outside the union, so it varies by location, specialty, sector, etc.