The way data is structured when shown differentiates Flat List reports from Hierarchy reports.
When creating a report, you can select the report type from the top drop-down in the configuration panel: Flat list, Hierarchy, Custom group, Segment compare, True path, and Click stream.
These reports show data in a flat structure. Flat list reports are ideal for analyzing a large data set. If you want to restrict the number of individual rows that are displayed, flat list reports offer options for a total row, plus another row. This option provides a summation of values that are not included in your Top N selection.
These reports show values for one display column based on the value of another display column. For example, you can view the most popular brands in each product category with the hierarchy report.