what is the difference between DBA_AUDIT_TRAIL and DBA_COMMON_AUDIT_TRAIL?

The difference between these views is that DBA_COMMON_AUDIT_TRAIL contains information from both standard and fine-grained auditing, while DBA_AUDIT_TRAIL only contains information from standard auditing.

DBA_AUDIT_TRAIL displays all standard audit trail entries. 

This view is populated only in an Oracle Database where unified auditing is not enabled. 

The SQL_BIND and SQL_TEXT columns are only populated if the AUDIT_TRAIL initialization parameter is set to 'db, extended'. 

DBA_COMMON_AUDIT_TRAIL displays 

• All standard audit trail entries 
• Fine-grained audit trail entries 
• Mandatory audit trail entries 
• SYS audit records written in XML format 

The SQL_BIND and SQL_TEXT columns are only populated if the AUDIT_TRAIL initialization parameter is set to 'db, extended' or 'xml, extended' or if the AUDIT_SYS_OPERATIONS initialization parameter is set to TRUE.