So what I want this to mean is that say you own EndWar2 and GRAW3. While playing EndWar2 say you have a mission that means take paris, well you use the map from EndWar2 and it saves it on you console, then it switches to GRAW to actually go in a have a looky lou and take paris. But if you also own SC or R6 you could do either of those options also, with each game offering unique mission in that scenario (obviously SC and GRAW can't have the same objectives/mission)
So I didn't do a great job of explaining what I mean, but it makes sense in my head.
But in fact it will probably just be squad-based (using the very loosest definition of that term) Halo.