29 The closer collaborators of the bishop are those priests who are charged with a pastoral office or apostolic organizations of a supra-parochial nature, whether in a certain area of the diocese or among special groups of the faithful or with respect to a specific kind of activity. Priests assigned by the bishop to various works of the apostolate, whether in schools or in other institutions or associations, contribute an exceedingly valuable assistance. Those priests also who are engaged in supra-diocesan works are commended to the special consideration of the bishop in whose diocese they reside, for they perform outstanding works of the apostolate.
Source: Christus Dominus (Vatican.va)