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 126.96.36.199 (Special Pathname Component Values) and Section 188.8.131.52 (Restrictions on Constructing Pathnames).
Other symbols and integers have implementation-defined meaning.
184.108.40.206.7 Notes about the Pathname Version Component
220.127.116.11 Restrictions on Constructing Pathnames