compatible parameter

What Is Oracle Database Compatibility?
Databases from different releases of Oracle Database software are compatible if they support the same features and those features perform the same way. When you upgrade to a new release of Oracle Database, certain new features might make your database incompatible with your earlier release.
Your upgraded database becomes incompatible with your earlier release under the following conditions:
A new feature stores any data on disk (including data dictionary changes) that cannot be processed with your earlier release.
An existing feature behaves differently in the new environment as compared to the old environment.

Checking the Compatibility Level of Oracle Database
The compatibility level of your database corresponds to the value of the COMPATIBLE initialization parameter.
SQL> SELECT name, value FROM v$parameter
WHERE name = 'compatible';

When to Set the COMPATIBLE Initialization Parameter in Oracle Database
Oracle recommends increasing the COMPATIBLE parameter only after complete testing of the upgraded database has been performed. After the upgrade is complete, you can increase the setting of the COMPATIBLE initialization parameter to the maximum level for Oracle Database 12c. However, after you increase the COMPATIBLE parameter, you cannot subsequently downgrade the database.