From what I observed, most positions are first offered as casual. Once you have been accepted and are part of the union, you can then apply for other positions.
Answered - Audiometric Technician 1 (Former Employee) - Williams Lake, BC
From what I understand, people are hired initially as casual workers, which gets them in the union, so they must have the qualifications a full time worker would. However, they can be fired more easily if they are "not a good fit" (read: "they don't cooperate with the abuse"), and they can be pressured to do full time hours without receiving full time benefits. But once they are in and have some seniority, they have better access (in theory) to the better union positions.
Answered - nurse (Former Employee) - Invermere, BC