Recovery volumes or .rev files are special files which can be created by WinRAR and allow to reconstruct missing and damaged files in a volume set. They can be used only with multivolume archives.
This feature may be useful for backups or, for example, when you posted a multivolume archive to a newsgroup and a part of subscribers did not receive some of the files. Reposting recovery volumes instead of usual volumes may reduce the total number of files to repost.
Each recovery volume is able to reconstruct one missing RAR volume. For example, if you have 30 volumes and 3 recovery volumes, you are able to reconstruct any 3 missing volumes. If the number of .rev files is less than the number of missing volumes, reconstructing is impossible. Total number of usual and recovery volumes must not exceed 255 for RAR 4.x and 65535 for RAR 5.0 archive format. Number of recovery volumes must be less than the number of RAR volumes.
Original RAR volumes must not be modified after creating recovery volumes. Recovery algorithm uses data stored both in REV files and in RAR volumes to rebuild missing RAR volumes. So if you modify RAR volumes, for example, lock them, after creating REV files, recovery process will fail.
Additionally to recovery data, RAR 5.0 recovery volumes also store service information such as checksums of protected RAR files. So they are slightly larger than RAR volumes which they protect. If you plan to copy individual RAR and REV files to some removable media, you need to take it into account and specify RAR volume size by a few kilobytes smaller than media size.
WinRAR reconstructs missing and damaged volumes either when clicking on .rev file, or when using rc command or automatically, if it cannot locate the next volume and finds the required number of .rev files when unpacking.
You may use "Recovery volumes" option of Archive name and parameters dialog or the similar option in Protect archive command to create recovery volumes. In the command line mode you may do it with -rv switch or rv command.
Original copies of damaged volumes are renamed to *.bad before reconstruction. For example, volname.part03.rar will be renamed to volname.part03.rar.bad.