A calling tree was a system used by pilots to communicate. Every ship in the tree must be in contact with two other ships, and those ships would be in contact with two other ships. This would allow a whole convoy to remain in constant contact, especially when under attack by an enemy force. After the Destruction of Sernpidal, Han Solo set up a calling tree with the convoy fleeing the planet, to better defend against its Yuuzhan Vong destroyers, and their swarms of bugs.