Characteristics
Apostolic administrators of stable administrations are equivalent in canon law with diocesan bishops, meaning they have essentially the same authority as a diocesan bishop. This type of apostolic administrator is usually the bishop of a titular see.
Administrators sede vacante or sede plena only serve in their role until a newly chosen diocesan bishop takes possession of the diocese. They are restricted by canon law in what they can do to the diocese they temporarily administer. For example, such an administrator may not sell real estate owned by the diocese. This type of administrator is commonly an auxiliary bishop of the diocese, a priest serving as the vicar general of the diocese, or the ordinary of a neighboring diocese.
Normally when a diocese falls vacant a vicar capitular/diocesan administrator is chosen locally, but the Pope, having full governmental power, can preempt this choice and name an apostolic administrator instead. Sometimes a retiring bishop is designated to be apostolic administrator until his successor takes office.
Read more about this topic: Apostolic Administrator