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.

• You can specify the name of a version control system. This is useful if the fileset can be managed by more than one version control system, and Emacs fails to detect the correct one.
• Otherwise, if using CVS, RCS or SRC, you can specify a revision ID. If the fileset is modified (or locked), this makes Emacs commit the files with that revision ID. You can create a new branch by supplying an appropriate revision ID (Branches). If the fileset is unmodified (and unlocked), this checks out the specified revision into the working tree. You can also specify a revision on another branch by giving its revision or branch ID (Switching Branches). An empty argument (i.e., C-u C-x v v =RET=) checks out the latest (a.k.a. ``head'') revision on the current branch. Specifying revision ID in this manner is silently ignored by 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.