So in the Apocalypse reloaded book, there is a formation for Chaos Space Marines called the Lords of the Black Crusade, which involves Abbadon, four Chaos Lords/Sorcerers and a squad of Terminators or Chosen. The rules say that "in battle, the Lords fight as one unit."

My question is, does this mean all the models are one unit, or just that Abbadon and the four Lords have to be one unit?