Unlike the model of America's commercial television networks, in which affiliates give up portions of their local advertising airtime in exchange for network programming, PBS member stations pay substantial fees for the shows acquired and distributed by the national organization.

This relationship means that PBS member stations have greater latitude in local scheduling than their commercial counterparts. Scheduling of PBS-distributed series may vary greatly from market to market. This can be a source of tension as stations seek to preserve their localism and PBS strives to market a consistent national line-up. However, PBS has a policy of "common carriage" requiring most stations to clear the national prime time programs on a common schedule, so that they can be more effectively marketed on a national basis.

Source: Wikipedia.org