Define agency shop.

An agency shop is characterized by the requirement that employees who are represented by a union, but who do not wish to join, still contribute to the costs associated with representing them. This typically means that non-union employees must pay dues that are equivalent to what union members pay, covering the bargaining and representation costs provided by the union. The rationale behind this arrangement is that all employees in a bargaining unit benefit from the collective bargaining activities of the union, and therefore, it is deemed fair that they contribute financially to these efforts.

This model seeks to avoid the "free rider" problem, where employees might benefit from union representation without paying for it. By ensuring that all employees share the financial burden, the agency shop encourages solidarity and supports the union's bargaining power.

The other options describe different concepts within union membership and labor relations. A requirement for all employees to join the union relates to a closed shop, while a type of union representing only select employees typically refers to craft unions or exclusive representation. A model that supports only voluntary union membership aligns with an open shop scenario, which does not require union membership as a condition of employment.