Complex sentence: An apostolic vicariate is to be distinguished from an apostolic prefecture, a similar type of territory that is usually under the leadership of a priest, which is not yet organised enough to be termed an apostolic vicariate.
Split into simpler sentences: An apostolic vicariate is to be distinguished from an apostolic prefecture, a similar type of territory that is usually under the leadership of a priest. The latter is not organised enough to be termed an apostolic vicariate.