IFF is an abbreviation for Identify Friend or Foe. Every ship in known space carries an IFF transponder on board to identify it to others in a given area. It is used by military forces across known space to distinguish friendly ships from foes, and forms the method by which the Friend-or-Foe missile locates targets.
It is possible to falsify IFF transponder signals, as has been done several times by Terran Confederation forces. Notably, the TCS Concordia transmitted a fake IFF signal at the conclusion of the First Battle of Kilrah in the Baragh System in 2667. It used a Kilrathi IFF code to sneak into the system to provide cover for the retreating TCS Tarawa. Also, Confederation Marines used a last minute switch to a Kilrathi IFF to board the Hakaga-class super carriers during the Battle of Earth in 2668.