The process was, as we were informally told during orientation, that "Preview" means some SVP thinks this has a chance of going over the minimum threshold of 200M worldwide users and is willing to put his weight behind that product, and "GA" means the product has gone over that threshold and will be "supported" until it goes back below the threshold at which point competing SVPs will start sharpening their knives, preparing to stick them in the back of the SVP that is currently in charge of the product.