Your qualifications are kinda saying the same thing as mine, though I guess I forgot to add in that of course you can have stuff from Imperial Armour and other FW stuffs as well as the new tend to make online dataslates and WD articles and all that what is also the same Faction.
The FACTION normally = the Book / Supplement that you are taking from
OK, maybe I'm misunderstanding your statement then, if you think we're saying the same thing. I'm not talking about IA or dataslates or any additional stuff. I'm talking about a single Combined Arms Detachment. I'm reading your statements as saying that a CAD must come from a single source, either codex or supplement. I'm saying that a CAD must all come from a single faction and that if there are multiple sources within that faction ie a codex and two supplements, then you can have units from all three sources in a single CAD (although you must still abide by any codex or supplement restrictions.
A good example is the Tau codex and Farsight supplement. You can have a CAD with units from both, but you must still meet the Farsight unit requirements and restrictions on units allowed. Or my IG codex and Stormtroopers supplement, I can take a Stormtroopers HQ and have a standard IG Platoon as a Troops unit in the same CAD because all the units are from the same Faction.