Another cool aspect is that <Keywords> only need to match within the detachment too, but this may effect detachment-wide rules that require "purity".
e.g. I run a Vanguard detachment of Adeptus Mechanicus with a Taurox Prime thrown in for fun (a dedicated transport from Astra Militarum). Since they share the <Imperium> keyword, I can you this, along with any other <Imperium> keyword models.
The downside is that Ad Mech canticles require all units be <Adeptus Mechanicus> for them to work, so that Vanguard detachment won't benefit from those. Also, the Taurox Prime's rules limit it's transport ability to <Astra Militarum Infantry> only, so it can't be used as a transport for my Ad Mech troops.
I receive 1 Command Point for the above. If I broke them out into a Vanguard (+1 CP) and an Auxiliary Support Detachment (-1 CP), the command points wash out but the Vanguard would benefit from Ad Mech Canticles.
A little confusing at first, but opens the door for some really personalized armies!