Originally Posted by
findark
Current templating uses Z for LUTXV and I for Q/K. If memory serves, L-differential fares used to book into I, and one of the non-public computer interfaces refused to honor companion certificates in Q or K differentialed to I (probably copied over the rules for C+).
However, if a revenue ticket prices in I or Z, this is sufficient to redeem a companion certificate. The only way to get hung up on this is to use a service like ExpertFlyer to check inventory explicitly, in which case you do need all of (1) I space, (2) Q space, and (3) a valid fare.
Perhaps I'm not doing things right, but I've had numerous times where I can find I or Z (when doing a mock booking for 2 seats without companion cert) and it shows like:
First (I)
or
First (Z)
and that itinerary NOT be available with the companion cert.
At least on Delta's website. Granted, last time I tried was last year.