Tranzit is for up to 4 players, Grief is for up to 8.
Tranzit is 1-4 player co-op. It will hold a majority of the EE and storylines (excluding future dlc) and it will also have waves like survival. Just think of it as a huge ass survival map. Survival maps will be specific locations from tranzit.(Diner, Farm, etc...) Grief will use the survival maps if I recall correctly... and that is 4v4vZombies.. Hope that clears up your confusion.
I was thinking grief should be using the big tranzit map since there will be 8 players...
if tranzit was only 1 player that would be kinda pointless becuase zombies is up to 4 players even though it has missions instead of one map.........but it also sucks becuase i dont have xbox live and i dont have the money for dlc content which is why they probably made tranzit and other maps to be actuallly zombies maps on the game instead of dlc content