When you give a prefix argument to
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 fileset is modified (or locked), this makes Emacs commit with that revision ID. You can create a new branch by supplying an appropriate revision ID (see Branches).
If the fileset is unmodified (and unlocked), this checks the specified revision into the working tree. You can also specify a revision on another branch by giving its revision or branch ID (see Switching Branches). An empty argument (i.e., C-u C-x v v <RET>) checks out the latest (head) revision on the current branch.
This is silently ignored on a decentralized version control system. Those systems do not let you specify your own revision IDs, nor do they use the concept of checking out individual files.