"Ecclesiae Sanctae", a 1966 Apostolic Letter of Pope Paul VI issued "motu proprio", directs that

Where a parish priest has been named to pastor a defined community, but circumstances do not permit it to be formally erected as a parish, the congregation is recognized as a "quasi-parish".Quasi-parishes would be found in new mission churches, called "missions" of the mother parish, in new neighborhoods, and in communities too small to support their own priest.