92 The civil authority itself constitutes the syndicate as a juridical personality in such a manner as to confer on it simultaneously a certain monopoly-privilege, since only such a syndicate, when thus approved, can maintain the rights (according to the type of syndicate) of workers or employers, and since it alone can arrange for the placement of labor and conclude so-termed labor agreements. Anyone is free to join a syndicate or not, and only within these limits can this kind of syndicate be called free; for syndical dues and special assessments are exacted of absolutely all members of every specified calling or profession, whether they are workers or employers; likewise all are bound by the labor agreements made by the legally recognized syndicate. Nevertheless, it has been officially stated that this legally recognized syndicate does not prevent the existence, without legal status, however, of other associations made up of persons following the same calling.
Source: Quadragesimo Anno (Vatican.va)