The New mode is always available in the check in dialog box.
This mode should be used in the following situations:
The document version you want to check in does not exist in the repository, or does not exist in the current branch (see Repository Branches) or its base branches. If the document exists elsewhere in the repository, a message box informs you that the new version will share the version tree of that document.
The document version you want to check in exists in the current branch, but you want to check it into another folder.