Elements inside custom groups act like separate and independent filters; they add different
GROUP BY clauses to the SQL statement generated by the report. The filtering condition can be an attribute qualification as in the previous example, a metric qualification, or other objects. Metric qualifications work like filters on metrics (see Chapter 4, Objects – Facts and Metrics, and Chapter 8, Dynamic Selection with Filters and Prompts, for more details).
I personally use custom groups because they offer a high level of flexibility and may be modified over time, when reporting requirements change, with no need to touch the database schema. As a plus, you can give different formats to different ...