Final answer:
An agency shop requires employees to pay union dues regardless of their union membership status, contrasting with a closed shop (union membership required) and an open shop (no union or dues requirement).
Step-by-step explanation:
The type of shop where employees are not required to join the union but are still required to pay union dues is known as an agency shop. Unlike in a closed shop, where employees must be members of the union to be hired, or a union shop, where employees must join the union shortly after being hired, in an agency shop, employees need not be union members. However, they must still pay fees to cover union services like collective bargaining. Comparatively, an open shop does not require union membership or dues payment. The American Plan heavily promoted the open shop concept. These practices are associated with historical labor movements and the opposition to compulsory unionism.