`UUID_SUB` shows the subvolume UUID (see [`man libblkid`][1] for details). For more on BTRFS subvolumes, see [the relevant section of the BTRFS documentation][2].


 [1]: https://man7.org/linux/man-pages/man3/libblkid.3.html
 [2]: https://btrfs.readthedocs.io/en/latest/Subvolumes.html