A third composite to unify two or more other composites?

somewhere in the userguide, maybe in a description to an example for composite builds, I think I have read about unifying two or more builds by means of a third actually empty composite whose existence serves the only purpose of doing this. I tried to find it but so far without success. Do you know this technique and do you know where it is described in the docs?
So project/composite C would require project/composites/builds A and B as dependencies.