Mixed mode auditing - 12c
In Oracle 12c Mixed Mode is enabled by default which makes the use familiar with Unified Auditing prior to switching to full Unified Auditing. Mixed Mode allows to pre-12c audit functionality to co-exist with new Unified Auditing functionality.

Using Mixed Mode audit data can be found both in the traditional locations as well as in SYS.UNIFIED_AUDIT_TRAIL.  This is because the Unified Auditing Policy ORA_SECURECONFIG is enabled by default.  

You can use the following commands to disable Unified Audit.

NOAUDIT POLICY ORA_SECURECONFIG;
noaudit policy ORA_LOGON_FAILURES;

Unified Auditing is not included in the Oracle 12c kernel so you have enable to manually. But in Mixed Mode it is on though it is not included in the kernel.
You can check whether the Unified Auditing is enabled or not using the following query.

select VALUE from V$OPTION where PARAMETER='Unified Auditing';
If you wish to linked the Unified Auditing to kernel then use the following commands.

cd $ORACLE_HOME/rdbms/lib
make -f ins_rdbms.mk uniaud_on ioracle ORACLE_HOME=$ORACLE_HOME

If you dont set AUDIT_TRAIL=NONE  then both auditing mechanisms will run concurrently.

Reference

Link to Oracle Official Documentation.

Mixed Mode Auditing