While the Query/400 product is an old work horse heading for retirement, it is still widely used in many shops around the world. For this reason, and even as more contemporary query and BI products are taking over, it is important to emphasize that as long as Query/400 is installed on a system it calls for consideration both in terms of operational and security aspects.
In order to address some of these requirements Query/400 has over time been subject to a number of enhancements delivered by means of special data areas. To document all these data areas and the features implemented, IBM has created a Technical Document, which in detail describes each data area, when it was introduced and exactly how it is used. You can find the document here.