From "man fstab"...
The fifth field, (fs_freq), is used for these filesystems by the
dump(8) command to determine which filesystems need to be dumped. If
the fifth field is not present, a value of zero is returned and dump
will assume that the filesystem does not need to be dumped.
The sixth field, (fs_passno), is used by the fsck(8) program to deter-
mine the order in which filesystem checks are done at reboot time. The
root filesystem should be specified with a fs_passno of 1, and other
filesystems should have a fs_passno of 2. Filesystems within a drive
will be checked sequentially, but filesystems on different drives will
be checked at the same time to utilize parallelism available in the
hardware. If the sixth field is not present or zero, a value of zero
is returned and fsck will assume that the filesystem does not need to