The version can be any symbol or any integer.
The symbol :newest refers to the largest version number that already exists in the file system when reading, overwriting, appending, superseding, or directory listing an existing file. The symbol :newest refers to the smallest version number greater than any existing version number when creating a new file.
The symbols nil, :unspecific, and :wild have special meanings and restrictions; see Section 18.104.22.168 (Special Pathname Component Values) and Section 22.214.171.124 (Restrictions on Constructing Pathnames).
Other symbols and integers have implementation-defined meaning.