This is from this → Steam Guide:
i : waves of specials
ii : monster specials
iii : hunting grounds
iv : sniper waves
v : only traitor guard faction (chaos faction are neutral and will still spawn (ogryns/pox))
vi : scarce ammo
vii : mutant waves (40% hp)
viii : Mostly Ranged Enemies
ix : Poxwalker burster waves
x :
xi :
xii : less intense, harder enemies
a : darkness
b : ventilation purge
c : melee only
d : extra barrels
e : nurgles blessing
f : 20% ability cdr
g : enhanced grenades (+2 nades, 50% cdr on brain burst)