A Dar archive stores information about its files at the end of the archive in a table of contents, or a "catalogue" (Both the author of Dar and the author of DarGUI are used to this spelling, so it tends to prevail..) When creating a differential backup based on an existing "reference" archive, Dar only needs to scan the reference archive's catalogue to determine changes in filesystem with respect to the reference archive.
DarGUI can "isolate" or extract the reference catalogue into a
separate file. The isolated catalogue can be read and listed like the
full reference archive but it contains none of the actual stored data.
Since the isolated catalogue is much smaller than the reference archive,
you can store it on your filesystem and use it for making differential
backups, i.e. use the isolated catalogue as your reference archive. To restore files, however, you need the original archive.
To isolate a catalog select Archive->Isolate Catalogue from the main menu. An dialog will appear which allows you to select an archive and the name to be given to the catalogue.
You can open the isolated catalogue in DarGUI and display the contents. The status field will display [InRef] to indicate that the actual file data is stored in another archive, the "reference archive". To restore the data you have to open the appropriate reference archive.
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