Because the header row of a group can appear multiple times when paging, the header templates are grouped by the occurrence of the header row, then by the templates of the header row, then by the column.
Usage:
For example, in the 1st header row that appears, the 2nd template of the 4th column is
It is important to note that both the value and range of the column index here is not the same as columnHeaders, as the header row includes templates for the top-left and top-right corner as well if applicable. The header rows collection also stores references to blank template instances used as place-holders when the template that would appear in that position is hidden.