Z is the fare class/bucket you upgrade into using a SWU. If your ticket says Z you are already as high as you can get.
BTW - welcome to FT!
On DL flights, Z is deep discount D1 or business class.
Let's not forget X and V fares, or S fares, so that the order after FULL Y should be as follows:
Any Y fare that isn't treated as FULL Y, B M S H Q K L U T X V award tickets.
The list doesn't list instrument supported instruments.
The new DL C+ (W) fares are obviously missing.