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The .r01 file extension belongs to the first compressed file (or sometimes it's .r00) in a collection (known as multi-volume) of files that make up a compressed archive, for example .r00 to .r40

These files are used to split-down large compressed files in to manageable chunks. This makes storing the file much easier, for example when trying to fit a 900mb file on to a 700mb CD.

The collection of files should include a file with the extension .SFV (Simple file verification) which checks that the correct number of files exist and that they have not changed since the multi-volume archive was created.