Originally Posted by
xliioper
It's LUTXV for the Plat (and Reserve if used for main cabin). For the Reserve card, It's the two lowest discount F classes (I and Z). I would first check if there is LUTXV on the flights you are looking at and, if not, see if there are other dates with availability if you have flexibility. If you find dates with LUTXV availability and the flights don't show when you try to use the cert, you might want to give Delta a call.
Can the Reserve be used for Z fares? It seems that every time I try it is bumping up to I (or previously A, from G), despite Z being readily available for sale. ISTR this was a point of contention that for FC you were pushed up from the lowest fare classes, vs. coach where the lowest of the low was what you were aiming for.
And indeed the T&Cs seem to indicate "AIWLUTXV" fare classes. https://www.delta.com/us/en/booking-information/companion-certificates
Tickets are only available in A and I classes of service for First Class travel, only available in W class of service for Delta Comfort+ travel, and only available in L, U, T, X, V classes of service for Main Cabin travel and may not be available on all flights or markets.