It really started with the v1.2 and the focus on 5 Chaos Legions: Alpha Legion, Death Guard, Emperor's Children, Iron Warriors and World Eaters. There are 5 Mods and each one is dedicated to one Legion including: New Models (Thanks to the Firestorm team for sharing those New Models). New Skins. New way to play.
Then, with the v1.4, came the 5 Space Marines Chapters facing the Legions. Now, each Chaos Legion can face one particular Space Marines Chapter. Alpha Legion Vs. Ultramarines. Death Guard Vs. Black Templars. Emperor's Children Vs. Imperial Fists. Iron Warriors Vs. Iron Hands. World Eaters Vs. Blood Angels.
All the Models and Skins are dedicated to each Legion and Chapter. There still are the restrictions about the different Chaos Gods. And some for the Space Marines Chapters. The CutterShane's Heroes Mod has been Integrated and tuned a little for the Mods. My Rotten's GainSomeRoom Mod has been added. The Chaos Fog of the original Chaos Buildings v1.0 is obviously present.