Do you understand how IFF works!!??First of all, IFF is not a radar mode
Secondly, it only helps to keep track of your own aircrafts, it doesn't help you to distinguish a target on the radar, that doesn't give away other signals to ID it. Civil airliners have transponders, to ID themselves to air traffic controllers, if that transponder fails, you have an unidentified target and send out fighters to ID or engage it if necessary.
If so can you explain it!!??