I saw that, but I'm still a bit confused. Is it saying that if you have two volumes using this system then one will be a copy of the other? Or some data will be

I saw that, but I'm still a bit confused. Is it saying that if you have two volumes using this system then one will be a copy of the other? Or some data will be on one volume and some will be on the other, but both volumes will look like they have all the data?