Application containers are new concept from 12.2 and they allow us to group PDB based on their purpose. Furthermore, we can treat these PDB as one whole in the way that we can partition among them, even if they are remote and thus increase the performance:

From 12.2 onward we are allowed to have a Proxy PDB, Application Root Container and a single user-defined PDB (regular or Application PDB) inside a single CDB without having to pay for the Multitenant Option. Notice we are still limited to a single user-defined PDB.

The management of an application root container is very similar to the management of a single PDB.

To create application root container, just create a normal PDB, but AS application root

CONN / AS SYSDBA

CREATE PLUGGABLE DATABASE appcon1 AS APPLICATION CONTAINER ADMIN USER app_admin IDENTIFIED BY Password1;
ALTER PLUGGABLE DATABASE appcon1 OPEN;
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To delete a application container, use the following command:

CONN / AS SYSDBA

ALTER PLUGGABLE DATABASE appcon1 CLOSE;
DROP PLUGGABLE DATABASE appcon1 INCLUDING DATAFILES;

To create an application PDB, we can use the same syntax as normal PDB, just we need to change the CDB :)

CONN / AS SYSDBA
ALTER SESSION SET container = appcon1;
CREATE PLUGGABLE DATABASE apppdb1 ADMIN USER pdb_admin IDENTIFIED BY Password1;
ALTER PLUGGABLE DATABASE apppdb1 OPEN;

To drop an application PDB, we can use the same command as a normal PDB

CONN / AS SYSDBA

ALTER PLUGGABLE DATABASE appcon1 CLOSE;
DROP PLUGGABLE DATABASE appcon1 INCLUDING DATAFILES;

Application containers and application roots are mostly useful when we install and deal with applications. But what is application in a DB term.