No idea honestly, I was mostly guessing after checking the STD battletome. I guessed it was about Marks/keywords like you said, so some units from STD can normally go to other armies, but not the other way around.
@Jaskier Ah, I see, thanks. Currently playing around with the list, thinking of running something like 3-4 DPs, Be'Lakor, a sorcerer, 40 marauders, 2 units of 5 knights, and a Soul Grinder, full Khorne (except sorcerer, duh). Considering sacrificing one of the DPs to make the sorcerer a Sorcerer on Manticore, since it looks cool as heck and also counts as a monster. Something like this, dunno if I'm going a bit too crazy or if this might surprise me in a good way O: