On Mon, 1 Oct 2018, Michael van Elst wrote:
ROOT=<part> only works for your special case where you know the root partition. An alias that represents device+partition would be more versatile.
The whole point of this is that the device is not known. Of course the partition layout is known, as the same tool that generated the image generated the fstab.
The mount by label scheme breaks down in the live image case where you are not given the opportunity to generate unique IDs for each partition. Being able to reference the root device in this way makes booting more robust.