When you have a database with 6 Billion dollars (US) flowing through it every year, you need to be able to account and prove exactly every single penny and every single action that occurred. So you better have _A tables for all of the main tables and have these columns to boot.

1. Create_user_id – Who/what created the record
2. Create_Dt – When exactly was this record created
3. Update_user_id – If updated, who updated this record (default null)
4. Update_Dt – when was it last updated (default null)
5. Archive_Dt – When can we legally destroy these records
6. Unique_Trans_id – So that tracing down everything that occurred becomes even easier.

It isn't sexy but it'll be handy if you ever need to trace down things in your database too.