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(emacs)Version Backups

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Version Backups
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   When VC sees that the CVS repository for a file is on a remote
machine, it automatically makes local backups of unmodified versions of
the file--"automatic version backups".  This means that you can compare
the file to the repository version (`C-x v ='), or revert to that
version (`C-x v u'), without any network interactions.

   The local copy of the unmodified file is called a "version backup"
to indicate that it corresponds exactly to a version that is stored in
the repository.  Note that version backups are not the same as ordinary
Emacs backup files (Note: Backup).  But they follow a similar naming
convention.

   For a file that comes from a remote CVS repository, VC makes a
version backup whenever you save the first changes to the file, and
removes it after you have committed your modified version to the
repository. You can disable the making of automatic version backups by
setting `vc-cvs-stay-local' to `nil' (Note: CVS Options).

   The name of the automatic version backup for version VERSION of file
FILE is `FILE.~VERSION.~'.  This is almost the same as the name used by
`C-x v ~' (Note: Old Versions), the only difference being the
additional dot (`.') after the version number.  This similarity is
intentional, because both kinds of files store the same kind of
information.  The file made by `C-x v ~' acts as a "manual version
backup".

   All the VC commands that operate on old versions of a file can use
both kinds of version backups.  For instance, `C-x v ~' uses either an
automatic or a manual version backup, if possible, to get the contents
of the version you request.  Likewise, `C-x v =' and `C-x v u' use
either an automatic or a manual version backup, if one of them exists,
to get the contents of a version to compare or revert to.  If you
changed a file outside of Emacs, so that no automatic version backup
was created for the previous text, you can create a manual backup of
that version using `C-x v ~', and thus obtain the benefit of the local
copy for Emacs commands.

   The only difference in Emacs's handling of manual and automatic
version backups, once they exist, is that Emacs deletes automatic
version backups when you commit to the repository.  By contrast, manual
version backups remain until you delete them.