The collection is made of a series of nested collections, grouping row footer templates into logical structures. It is first grouped by the logical item or row of items the footer is for, then by the sections of footer 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 footer template after the 1st child group in a non-wrapped vertically repeating
group would be