They were residential Latin bishops while the Latin kingdom lasted, then they were Latin cardinals in the Papal Court, who were also the cardinal archpriests of St. Peter's Basilica. When you hear St. Peter's being refered to as the Patriarchal Basilica of the Patriarch's of Constantinople this is that which is being referred. The unionist Greek Bishops Isidore of Kiev and John of Nicea were both given that title, in addition to that of cardianl, in succession, first Isidore, then upon his death, John.
Fr. Deacon Lance