Creating Objects in the System tablespace.
It has long been a DBA best practice to avoid creating objects in the SYSTEM tablespace. SYSTEM tablespace is for Oracle system objects only. Unfortunately, if an Oracle user account is created without specifying the default tablespace, any objects created will appear in SYSTEM.
The best practice is to associate each user account with a default tablespace.
See Also: TECH221780: Does Symantec DLP create objects in the SYSTEM tablespace?