Originally Posted by
Utskicat
They are just different buckets for different scenarios.
Upgrade availability is now based on revenue buckets J Z P G. But they don’t actually book into those codes. If availability is in the correct code then they map to a different segment booking code. If you look up at the top of this thread in the wiki the mapped classes are listed in a chart.
Your wiki entries seem to claim that RUCs can be used to upgrade into Premium Select (and from Premium Select to D1), but I don't believe this is the case. Nowhere does DL indicate that RUCs can be used for Premium Select. There are no Premium Select fares on routes where FC fares are sold, nor on domestic D1 routes (PS cabins on domestic D1 are sold as C+, not PS) which are the only routes where RUCs are applicable.