Source : Free On-Line Dictionary of Computing

IFF
     
        1.  {Interchange File Format}.
     
        2. Identify friend or foe (radar).

iff
     
         if and only if, i.e. necessary and
        sufficient.  For example, two figures are {congruent} iff one
        can be placed over the other so that they coincide.
     
        (2002-12-28)