The sixth field, (fs_passno), is used by the fsck(8) and quotacheck(8)
programs to determine the order in which file system and quota checks are
done at reboot time. The fs_passno field can be any value between 0 and
‘INT_MAX-1’.
The root file system should be specified with a fs_passno of 1, and other
file systems should have a fs_passno of 2 or greater. A file system with
a fs_passno value of 1 is always checked sequentially and be completed
before another file system is processed, and it will be processed before
all file systems with a larger fs_passno.
For any given value of fs_passno, file systems within a drive will be
checked sequentially, but file systems on different drives will be
checked at the same time to utilize parallelism available in the
hardware. Once all file system checks are complete for the current
fs_passno, the same process will start over for the next fs_passno.
If the sixth field is not present or is zero, a value of zero is returned
and fsck(8) and quotacheck(8) will assume that the file system does not
need to be checked.