Review to understand compatibility differences.

Compatibility and interoperability issues may arise because of differences between Oracle Database releases. These differences can affect both general database administration and existing applications.

• About Oracle Database Release Numbers
  Oracle Database releases are categorized by five numeric segments that indicate release information.
• Convention for Referring to Release Numbers in Oracle Database Upgrade Guide
  Review to understand how statements apply to releases.
• What Is Oracle Database Compatibility?
  If new features are incompatible with your earlier release, then Database compatibility can cause issues.
• What Is Interoperability for Oracle Database Upgrades?
  In the context of upgrading Oracle Database, interoperability is the ability of different releases of Oracle Database to communicate and work in a distributed environment.
• About Invalid Schema Objects and Database Upgrades
  Run utlrp.sql to validate invalid objects as part of your upgrade test plan.
• About Upgrading Oracle OLAP Data Security Policies
  Security policies are stored differently in Oracle Database 12c. You may need to carry out actions before and after upgrades.