Editing Warhammer/Tactics/6th Edition/Chaos (section)

===Marks of Chaos===
Many Chaos units and characters have Marks of Chaos. The default is the Mark of Chaos Undivided: all units which can have a Mark start off with this one, and it's free. You can choose to dedicate your units to a particular Chaos God and gain upgrades, for a few (or more than a few) extra points. The Marks do different things depending on the model they're applied to, and we'll get into the specifics of those later.

The important thing for the army as a whole is that if your General has a particular God's Mark, all your troops who can have a Mark must have the same one, or stick with Chaos Undivided. If your General has the Mark of Chaos Undivided, you can go hog wild with the rest of your units and characters and take what you like. 

Also, characters with a Mark can't join units with a different one, although they can join units with no Mark at all (which is how you get characters into Beast Herds, Marauders and other troops that don't have the option).