You can use nested regions when there is a relationship between data tables in the data source. Here is an example of a report template for listing product by category. There are two tables: Products and Categories linked by the CategoryID field. The outer region is for category description and the inner region lists products for each category:
Nested regions defined
The result is products grouped by categories: Nested regions in the final report
See how to define related tables in a report data source in the TableRelation element description.