What constitute "mandatory auditing" under Unified Auditing?

• Activities from administrative users such as SYSDBA, SYSBACKUP, and SYSKM. 
•  The following audit-related activities are mandatorily audited:

CREATE AUDIT POLICY	AUDIT	EXECUTE of the DBMS_FGA PL/SQL package
ALTER AUDIT POLICY	NOAUDIT	EXECUTE of the DBMS_AUDIT_MGMT PL/SQL package
DROP AUDIT POLICY	Access to sensitive columns in the optimizer dictionary tables.	ALTER TABLE attempts on the AUDSYS audit trail table
Top level statements by the administrative users SYS, SYSDBA, SYSOPER, SYSASM, SYSBACKUP, SYSDG, and SYSKM, until the database opens	All user-issued DML statements on the SYS.AUD$ and SYS.FGA_LOG$ dictionary tables	Any attempts to modify the data or metadata of the unified audit internal table. SELECT statements on this table are not audited by default or mandatorily.
All configuration changes that are made to Oracle Database Vault

The audit information can be found in the view UNIFIED_AUDIT_TRAIL.
Documentation for Mandatory Unified Auditing in Oracle 12.2 can be found here