* Example, Creating Common User in a CDB
This example creates the common user c##testcdb.
CREATE USER c##testcdb IDENTIFIED BY password DEFAULT TABLESPACE cdb_tbs QUOTA UNLIMITED ON cdb_tbs CONTAINER = ALL; A common user's user name must start with C## or c## and consist only of ASCII characters. The specified tablespace must exist in the root and in all PDBs.
*CREATE USER with CONTAINER (optional) clause / CONTAINER = ALL Creates a common user. / CONTAINER = CURRENT Creates a local user in the current PDB.
*CREATE USER
*The following rules apply to the current container in a CDB:
The current container can be CDB$ROOT (root) only for common users. The current container can be a particular PDB for both common users and local users.
The current container must be the root when a SQL statement includes CONTAINER = ALL.
You can include the CONTAINER clause in several SQL statements, such as the CREATE USER, ALTER USER, CREATE ROLE, GRANT, REVOKE, and ALTER SYSTEM statements.
Only a common user with the commonly granted SET CONTAINER privilege can run a SQL statement that includes CONTAINER = ALL.

*Using any of the following DDL statements on a table enabled for Flashback Data Archive causes error ORA-55610:
ALTER TABLE statement that does any of the following:
Drops, renames, or modifies a column Performs partition or subpartition operations Converts a LONG column to a LOB column Includes an UPGRADE TABLE clause, with or without an INCLUDING DATA clause DROP TABLE statement
RENAME TABLE statement
TRUNCATE TABLE statement
*After flashback archiving is enabled for a table, you can disable it only if you either have the FLASHBACK ARCHIVE ADMINISTER system privilege or you are logged on as SYSDBA. While flashback archiving is enabled for a table, some DDL statements are not allowed on that table.

AB: You can make individual table columns invisible. Any generic access of a table does not show the invisible columns in the table. For example, the following operations do not display invisible columns in the output:
*SELECT * FROM statements in SQL
*DESCRIBE commands in SQL*Plus
*%ROWTYPE attribute declarations in PL/SQL
*Describes in Oracle Call Interface (OCI)
Incorrect:
Not D: You can make invisible columns visible.
You can make a column invisible during table creation or when you add a column to a table, and
you can later alter the table to make the same column visible.
Reference: Understand Invisible Columns