When you give a prefix argument to vc-next-action (C-u
C-x v v), 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 revision ID
to use for the new version that you commit. This is one way to create
a new branch (see Branches).
If the file is not modified (and unlocked), you can specify the
revision to select; this lets you start working from an older
revision, or on another branch. If you do not enter any revision,
that takes you to the highest (“head”) revision on the current
branch; therefore C-u C-x v v <RET> is a convenient way to
get the latest version of a file from the repository.
Instead of the revision ID, 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 Local Version Control).