The collection is made of a series of nested collections, grouping row header templates into logical structures. It is first grouped by the logical item or row of items the header is for, then by the sections of header templates that correspond to interleaved child groups, then by the templates of each section.
Usage:
For example, on the 8th row of items, the 2nd header template after the 1st child group in a non-wrapped vertically repeating
group would be