A Papal Nuncio (also known as an Apostolic Nuncio) is a permanent diplomatic representative (head of mission) of the Holy See to a state, having ambassadorial rank.
Internuncios (also heads of missions) rank below nuncios, being of the rank of an envoy or minister, while a legate a latere is a temporary papal representative or a representative for a special purpose.
A Pro-Nuncio is a diplomatic representative who is not of full ambassadorial rank, his status representing the nature of the diplomatic relationship between the Holy See and the state to which he has been assigned. Until recent times a Pro-Nuncio served as the papal representative to the United States and the United Kingdom. Both states in the late twentieth century upgraded their relationship with the Holy See to full ambassadorial level, allowing for the appointment of a papal nuncio.
The Vienna Convention on Diplomatic Relations provides that (article 14(2)) "Except as concerns precedence and etiquette, there shall be no differentiation between heads of mission by reason of their class." Some states give special precedence to representatives of the Holy See, which they are permitted to do by the Convention (article 16(3)) "This article is without prejudice to any practice accepted by the receiving State regarding the precedence of the representative of the Holy See."
See also: Foreign relations of the Vatican City