So, yeah, you have to be really careful with your wording.
Your army can have up to three detachments. A Formation is a specific type of detachment, but you can't duplicate formations or detachments. So, no double CAD. Also, you can't do two of the same Formation.
How does the Daemonkin formation work? What's it made of? Like "War Engine" -- is that a formation or a data slate or... what's it called in the book?