An ordinariate is a form of ecclesial jurisdiction between a vicariate and a diocese or eparchy. Vicariates and ordinariates are used under circumstances in which the ordinary diocesan form of jurisdiction is not suitable. In a vicariate, which is most commonly used where there is no diocese at all, the whole is supervised by a vicar (whether apostolic or patriarchal), who is a bishop who is simply delegated the care of the faithful in question on top of any other duties he might have; the vicar is not an ordinary, and thus is simply administering matters until they develop enough to be given a more formal organization. (There are even less developed structures, prefectures, that are headed by people who do not have episcopal functions at all.) Ordinariates differ from vicariates in that they are supervised by an ordinary, someone whose primary authority and responsibility is the care of the faithful who are part of the ordinariate. The most common ordinariates with which anyone deals are military ordinariates. Unlike a vicariate, an ordinariate functions very much like a diocese in its own right; the ordinary is not necessarily a bishop, and can be just a priest, but in terms of his authority and jurisdiction, he is not fundamentally different from a bishop.
There are a number of areas of the world in which there are many Eastern Catholics, but they may be scattered in such ways, or in such circumstances, that it is difficult to organize an actual eparchy for them. Thus there have come to be what are known as Ordinariates for the Faithful of Eastern Rite. Three of them are part of the Armenian Catholic Church; two of these are the oldest such ordinariates in existence (dating from 1925 and 1930), and they arose because of the peculiar circumstances involved in the origin of the Armenian Catholic Church, which was consolidated out of a number of different Armenian groups that came into union with Rome. Some pockets were not in situations in which an eparchy could be easily developed, and it eventually was decided that they were more adequately taken care of under their own structures rather than under Latin bishops exercising their ordinary diocesan functions.
The rest of the ordinariates for the faithful of Eastern rite, however, are not distinctive to a particular church. One of them is for Byzantine Rite Catholics in Austria. The other four, in Argentina, Brazil, France, and Poland, are for Eastern Catholics in general. In none of these last four cases, however, are all Eastern Catholics included: the ordinariates exist for situations in which there is no proper eparchial structure. In the one for Brazil, for instance, the Ukrainians, Maronites, and Melkites are all able to maintain their own eparchies for their particular faithful, and thus they are cared for by those eparchies rather than the ordinariates. In each case, the most important Latin Archbishop in the territory is the ordinary for the ordinariate, and thus in practice they serve as means whereby the Latin Church may help to serve the spiritual needs of Eastern churches, where it is in a better position to help them than the relevant Eastern church.