UUID_SUB shows the subvolume UUID (see man libblkid for details). For more on BTRFS subvolumes, see the relevant section of the BTRFS documentation.
Every BTRFS filesystem has at least one subvolume:
A freshly created filesystem is also a subvolume, called top-level, internally has an id 5. This subvolume cannot be removed or replaced by another subvolume. This is also the subvolume that will be mounted by default, unless the default subvolume has been changed (see btrfs subvolume set-default).
(from BTRFS manual on subvolumes)
