As the master complainer of NW's stinginess of award seats in the year 2000, I'll respond:

J = premium business class inventory
C = discount (if you really want to call it that) business class inventory; rulebuster award bucket inventory
Z = upgrade bucket inventory for full-fare; unpublished business class fare inventory (for consolidators)
I = upgrade bucket inventory for non-full fare
D = upgrade bucket inventory for non-full fare and standard award bucket inventory