/etc/mtab file shares the same structure as
/etc/fstab. According to this site the 5th and 6th column in
/etc/fstab are used to store "Dump and fsck options". The 5th column is used to determine if dumping of the partition should be made, and the 6th to decide if an fsck must be processed on the partition.
/etc/mtab, however, this two options loose their sense. Indeed, these two options are used when mounting the partitions, and
/etc/mtab lists the partitions that are already mounted. If I understand it correctly, these option are not useful in
/etc/mtab. They may be here for compatibility reasons with
/etc/fstab, as the content of
/etc/mtab must be directly usable in