Classic uses the versioning approach Major.Minor.Patch.
However due to the nature of theme’s the interpretation of these number is slightly different.
Classic will update the Patch number when adding to the content area’s of the theme. I.e. posts or pages. There is no need to follow these updates closely for your own theme.
Classic will update the Minor number when bugfixes or minor additions are made. A list of changed files will be provided. It may be necessary to re-apply changes that were made to the theme’s files at your end.
Classic will update the Major number when significant additions have been made. The difference between minor and major additions is entirely subjective.
Backwards compatibility with previously used YAML tags is a very important driver for further development.