GNU Emacs Manual
M.7.3.3 Advanced Control in C-x C-q
When you give a prefix argument to
C-x C-q), it still performs the next logical version control
operation, but accepts additional arguments to specify precisely how
to do the operation.
If the file is modified (or locked), you can specify the version
number to use for the new version that you check in. This is one way
to create a new branch (see section M.7.6 Multiple Branches of a File).
If the file is not modified (and unlocked), you can specify the
version to select; this lets you start working from an older version,
or on another branch. If you do not enter any version, that takes you
to the highest version on the current branch; therefore C-u C-x
C-q RET is a convenient way to get the latest version of a file from
Instead of the version number, you can also specify the name of a
version control system. This is useful when one file is being managed
with two version control systems at the same time (see section M.7.7.2 Local Version Control).