Although it usually has an (embryonal) see, it is often not called after such city but rather after a natural feature, or administrative geographical area, which may be a name in use by the local inhabitants, or one assigned by a colonial authority, depending on the circumstances under which the prefecture was established.
Both these stages remain missionary, hence exempt, that is, directly subject to the Holy See, specifically the Dicastery for Evangelization, rather than, as a diocese normally would, belong to an ecclesiastical province.
During the last centuries of the second millennium it was the practice of the Holy See to govern either through prefects apostolic or apostolic vicariates, many territories where no dioceses with resident bishops were erected and where local circumstances, such as the character and customs of the people or hostility of civil powers, made it doubtful whether an episcopal see could be permanently established.
When a vicariate or a diocese extended over a very large territory in which the Catholic population was unequally distributed, the Holy See sometimes placed a portion of the territory in charge of a prefect apostolic; in which case the faculties of the prefect were more limited, and in the exercise of his office he was supervised by the vicar apostolic or the diocesan bishop.
[1] With a view to better protecting the authority of the local vicar apostolic or bishop, it was proposed in the First Vatican Council to abolish prefects apostolic having jurisdiction over districts within a vicariate or diocese of the Latin Church, but the Council was interrupted and the practice continued until Pope Leo XIII abolished them within the Oriental Churches by a decree of Propaganda Fide on 12 September 1896, and established superiors with special dependence on the papal representatives of the areas concerned.