System or audit journal (QAUDJRN) is a journal that in charge to record all activities of the system based on the settings of the QAUDLVL and QAUDLVL2 system values. What is not recorded by QAUDJRN is only database activity. Examples of records are delete, create and change object. System journal only one in the AS400 system and is always in the library QSYS with the journal name QAUDJRN. How to make QAUDJRN can be seen in this article: Create AS400 Audit Security Journal
User journal are journals whose job is to record all the journaled database activities. All data activities such as insert, delete or update records are recorded in this journal. User journaling can be more than one on the AS400 system according to the user’s requirements. How to create a user journal can be seen in this article: Creating AS400 User Journal
So when going to do an audit, we already know where to check if you use journal. For auditing system activities, we use QAUDJRN while for database activities we use user journals.