In some industries, most jobs are filled through referrals from union hiring halls.
Employers in the construction and maritime industries often choose to hire exclusively through referrals from union hiring halls. Unions that operate exclusive hiring halls must notify workers how the referral system works (and of any changes in that system) and maintain non-discriminatory standards and procedures in making job referrals from the hiring hall. You don't have to be a union member to use a hiring hall and a union may not discriminate in making referrals based on whether or not you are a union member. It may, however, charge nonmembers a reasonable fee to use the hiring hall's services.