Most version control systems allow you to apply a revision tag to a specific version of a version-controlled tree. On modern changeset-based version control systems, a revision tag is simply a symbolic name for a particular revision. On older file-based systems like CVS, each tag is added to the entire set of version-controlled files, allowing them to be handled as a unit. Revision tags are commonly used to identify releases that are distributed to users.
There are two basic commands for tags; one makes a tag with a given name, the other retrieves a named tag.
C-x v sname
C-x v rname
This command reports an error if any files are locked at or below the current directory, without changing anything; this is to avoid overwriting work in progress.
You can give a tag or branch name as an argument to C-x v = or C-x v ~ (see Old Revisions). Thus, you can use it to compare a tagged version against the current files, or two tagged versions against each other.
On SCCS, VC implements tags itself; these tags are visible only through VC. Most later systems (including CVS, Subversion, bzr, git, and hg) have a native tag facility, and VC uses it where available; those tags will be visible even when you bypass VC.
In file-based version control systems, when you rename a registered
file you need to rename its master along with it; the command
vc-rename-file will do this automatically
(see VC Delete/Rename).
If you are using SCCS, you must also update the records of the tag, to
mention the file by its new name (
vc-rename-file does this,
too). An old tag that refers to a master file that no longer exists
under the recorded name is invalid; VC can no longer retrieve it. It
would be beyond the scope of this manual to explain enough about RCS
and SCCS to explain how to update the tags by hand. Using
vc-rename-file makes the tag remain valid for retrieval, but it
does not solve all problems. For example, some of the files in your
program probably refer to others by name. At the very least, the
makefile probably mentions the file that you renamed. If you retrieve
an old tag, the renamed file is retrieved under its new name, which is
not the name that the makefile expects. So the program won't really
work as retrieved.