Administrators of Prince-bishoprics
Since the Investiture Controversy in 11th and 12th the cathedral chapters used to elect the Catholic bishops in the Holy Roman Empire. Prince-bishoprics were elective monarchies of imperial immediacy within the Empire, with the monarch being the respective bishop usually elected by the chapter and confirmed by the Holy See, or exceptionally only appointed by the Holy See. Papally confirmed bishops were then invested by the emperor with the princely regalia, thus the title prince-bishop. However, sometimes the respective incumbent of the see never gained a papal confirmation, but was still invested the princely power. Also the opposite occurred with a papally confirmed bishop, never invested as prince.
Candidates elected, who lacked canon-law prerequisites and/or papal confirmation, would officially only hold the title diocesean administrator (but nevertheless colloquially be referred to as prince-bishop). This was the case with Catholic candidates, who were elected for an episcopal see with its revenues as a mere appanage and with all Protestant candidates, who all lacked either the necessary vocational training or the papal confirmation.
With many capitulars converting to Lutheranism or Calvinism during the Reformation, the majorities in many chapters consisted of Protestant capitulars. So they then also elected Protestants as bishops, whom usually were denied papal confirmation. However, in the early years of Reformation, with the schism not yet fully implemented, it was not always obvious, who tended to Protestantism, so that some candidates only turned out to be Protestants after they had been papally confirmed as bishop and imperially invested as prince. Later, when Protestants were usually denied papal confirmation, the emperors nevertheless invested the unconfirmed candidates as princes - by a so-called liege indult (German: Lehnsindult) - due to political coalitions and conflicts within the empire, in order to gain candidates as imperial partisans.
Many Protestant candidates, elected by the capitulars, neither achieved papal confirmation nor a liege indult, but nevertheless, as a matter of fact held de facto princely power. This was because the emperor would have to use force to bar the candidates from ruling, with the emperors lacking the respective power or pursuing other goals. A similar situation was in a number of imperially immediate abbeys with their prince-abbots and princess-abbesses.
As administrators the incumbents of the sees had comparable power like any other monarch of an imperially immediate territory within the Empire, just being elected instead of having succeeded by way of inheritance. However, one common restriction was that administered prince-bishoprics were denied to emit their deputees to the diets of the Empire or of the imperial circles (German: Reichstag, or Kreistag, respectively). This restriction was abandoned by the Peace of Westphalia in 1648, when the emperor accepted Protestant administrators as fully empowered rulers. However, the Peace also secularised many of the prior Protestant prince-bishoprics and transformed them into hereditary monarchies.
Read more about this topic: Diocesan Administrator