Cool! So in other words, you can build a Battle-forged army from various Factions as long as they are all Imperium! Obviously I doubt you can build a xenos army of Tau and Orks, but for Imperium players, this is great news! That means I can field a combined Skitarii-Imperial Guard army. For example, Skitarii Vanguard as Troops and Leman Russ tanks as Heavy Support! And still have space for Onager Dunecrawlers! It would be great to field a Super-heavy Deatchment of a Stormsword and a couple of Imperial Knights as well!
There's lots of ways you can build it, and it's very flexible. For example, you want Imperial Guard Leman Russ tanks, but you don't want their infantry. Then take a Grey Knights Terminator squad for Infantry! An Adeptus Mechanicus Tech-Priest Dominus as your HQ to repair those tanks! Stuff like that!
If course, the flipside is that I get more bonuses if I field a complete Imperial Guard army as opposed to my Skitarii-Imperial Guard hybrid army, but we shall see. Anyway, I think this is good news and allows for a ton of flexibility yet doesn't get too overpowered because you don't get the bonuses, unlike Formations of the past where you just take whatever Formations and group them together for the various bonuses. This edition is shaping up to be really awesome!