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.
Otherwise, if using CVS, RCS or SRC, you can specify a revision ID.
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.