Here is what the main report can look like with the drilldown fields selected.
You may now interact with your main report to view your sub-report. When you click on Client Code, you will notice that the sub-report filters data by both Client Code and Client Name. If you were to use data in a many-to-many relationship, only data that satisfied those two conditions would be shown.
Now when you click on the Client Code column, the result will be filtered by Client Code and Client Name. In the result below, Client Code 000286 was clicked.