The Hierarchy is: Enemy Group Than Faction.
Case-in-point. There is a (Enemy) Group called "Vanguard" but they have a mercenary faction within the (enemy) group you actually get missions to take them down.
There are a few other examples, but I'm probably just going to start another debate even bringing this up.
In-Game:
Enemy Group is the the generalized mass of NPCs with the same common goal,
Factions are sub-groups of the same Enemy Group that have splintered off in an attempt to complete their own belief of said goal.
BUAH HAHAHA, Steiner actually knew something for once...
Cue someone to come in and correct in 3...2...
