Complex sentence: Many extant viscountcies are used as courtesy titles; a specifically British custom is the heir apparent of an earl or marquess being referred to as a viscount, if the second most senior title held by the head of the family is a viscount.
Split into simpler sentences: Many extant viscountcies are used as courtesy titles; a specifically British custom is the heirs of an earl or marquess who hold a viscountcy using the it as a courtesy title. The peer's heir apparent will sometimes be referred to as a viscount, if the second most senior title held by the head of the family is a viscount.