The map usedinomap is a list of inodes that have been deleted since the last dump. restore uses this map to delete files before doing a restore of files in this dump. The map dumpinomap is a list of all inodes contained in this dump. Each header contains a lot of information:

Record type
Dump date
Volume number
Logical block of record
Inode number
Magic number
Record checksum
Inode
Number of records to follow
Dump label
Dump level
Name of dumped filesystem
Name of dumped device
Name of dumped host
First record on volume

TS_TAPE
     dump header
TS_CLRI
     Map of inodes deleted since last dump
TS_BITS
     Map of inodes in dump
TS_INODE
    ...

When dump writes the header, it includes a copy of the inode for the file or directory that immediately follows the header. Since inode data structures have changed over the years, and different filesystems use slightly different inode data structures for their respective filesystems, this would create a portability problem. So dump normalizes its output by converting the current filesystem's inode data structure into the old BSD inode data structure. This BSD data structure is written to the backup volume.

As long as all dump programs do this, then you should be able to restore the data on any Unix system that expects the inode data structure to be in the old BSD format. It is for this reason you can interchange a dump volume written on Solaris, HP-UX, and AIX systems.

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