Unions are membership-based organizations. However, unions in Canada are unlike other membership-based SMOs two ways:
Union membership is not always voluntary. Most workers in Canada are in a union because the job they were hired into was already unionized.
Union membership involves a legal relationship between members and between union members and employers that is regulated by legislation.
In Canada, unions are regulated by provincial and federal labour codes. These codes create legal rights and responsibilities that other social movement organizations do not possess, such as:
Unionized workers can legally withdraw their labour (strike) without being fired or arrested.
Unions are obliged to support all members equally, and to negotiate on their behalf, even if those members act against the union.