The RAR filename extensions are .rev for recovery volume set and .rar for data volume set. Older RAR versions split bigger archives into many smaller files and make a "multi-volume archive". Also, the numbers were utilized in the smaller file's file extensions for keeping them in an accurate order. The first file utilized the .rar extension, then the .r00 extension for the second, and then the .r01, .r02 extensions, etc.