sofasurfer,
Your question appears to be more about incremental backups than general tar usage as your subject implies. Just to be clear, the main use of tar is to build archives for distributing files such as programs. If you want to do that, you don't need to care about snar files at all.
Incremental backups are used for archiving data repeatedly, to avoid having to rebuild the entire archive all the time and reduce archive size.
A level 0 backup is basically an archive (.tar) file, and just contains the current state of the files, and puts additional information into a separate archive (.snar) file.
A level 1 backup is just a normal archive (.tar) file too, but any files that haven't changed since the level 0 backup was made aren't included.
If you use --incremental after a level 0 backup, then it will create a level 1 archive, but it won't create a new (.snar) file, so you any subsequent backups based on this archive. This is useful when you have a directory containing files that need to be archived, but rarely change.
If you use --listed-incremental after a level 0 backup, it will also create a level 1 archive. It will also create a new (.snar) file. This means that any subsequent backups can be based on this archive. This is useful when you want to archive a directory containing files which change frequently.
You should use different filenames for each snar file. The examples at
http://www.gnu.org/software/tar/manu...tal-Dumps.html use a -
level postfix to the filename (.snar, .snar-1, etc).
Hope that helps,
—Robert J Lee