Emergency Code Zero was a designation used by the Imperial forces to signify an urgent situation. Any ship that received this signal was required to immediately change its course and provide assistance.
During the Space Battle at Kuat, this code was put into effect when Rebellion vessels launched an assault on a shipyard orbiting the planet. As a consequence of the code's activation, the Star Destroyer called Tyranny was pulled away from its assignment at Fresia, a position to which it had been assigned by Darth Vader.