Providing a book asset with a version string is what makes a versioned book. The version string is supplied within a meta element in the package document for the book asset.
For new books, versioning is optional. Previously-published books that used version numbers are required to continue to use version numbers.
Note: Sample assets cannot be versioned. Sample files that contain version numbers are blocked from delivery. Samples inherit the version number of the source asset. If the previous book version included a custom sample and you do not deliver a replacement sample, the old sample is used. If the previous book version did not include a custom sample and you do not deliver one, a new sample is generated.