The sixth field (fs_passno).
      This field is used by fsck(8) to determine the order in which filesystem  checks  are  done  at  boot
      time.   The root filesystem should be specified with a fs_passno of 1.  Other filesystems should have
      a fs_passno of 2.  Filesystems within a drive will be checked sequentially, but filesystems  on  dif‐
      ferent  drives  will  be  checked  at the same time to utilize parallelism available in the hardware.
      Defaults to zero (don't fsck) if not present.