19c Major Release
The Release is the major number, and should be released annually going forward. This is where the majority of new features are introduced to the database.
19.x - Release Update (RU).
The RU includes function fixes, security fixes, optimizer changes, functional enhancements and emergency patches. RUs should be released for about two years after the initial release date of the database. Most development environments should patch at the RU level. The RU replaces the older Bundled Patch (BP).
19.x.x - Release Update Revision (RUR)
Focuses on security and regression patches only and would not normally change other aspects of the database.