Regular full cave allows deco and trimix, but how can you do deco and trimix if you don't have the training to do it? Full cave doesn't teach deco and trimix, so if you're not deco trained and certified already, then you end up cutting your dives short to stay within NDL if you're at places like Devil's, Little River, etc.
Since decompression is part of cave diving, it is meant to be taught by both NACD & NSS-CDS standards at the Full Cave Level.
People forget that decompression was traditionally taught during Full Cave class for many, many years as there were no other courses available out there, now of course there are a myriad of ways to spend your hard earned money!
If my Full Cave students already have Adv Nx/Deco procedures I make sure that they do actually know what we're talking about, but if they haven't taken a class, I can tell and show them what they need to know to decompress on a limited basis in the cave environment.......
As always YMMV......
Safe diving,
Rich