PDF/X-Plus Creation and Preflight specifications and Best Practices are the core work of the Ghent PDF Workgroup. They include documentation that explain the theoretical sides of the specifications and they include deliverable reference implementations.
Specifications created by the Ghent PDF Workgroup are the result of the work of the full workgroup and the specific subcommittees. They are adjusted / published, on average once a year.
The specifications in each yearly release are available from this section of the web site.