Very VERY
VERY VERY old, this one, and has been used around the world (as well as several times on this QF forum).
P: IFF inoperative.
S: IFF always inoperative in OFF mode.
IFF stands for Interrogation Friend or Foe, and is used by missile targetting systems to determine whether the target is a friend or an enemy.
The last time I looked, QF aircraft did not have missile deployment systems.
Dave