cd to your eZ publish root dir grep -irlP "grouped_related_contentobjects" *
you will get back a list with at least these files:
design/admin/templates/content/edit_relations.tpl
doc/features/3.5/related_object.txt (contains the documentation) kernel/content/relation_edit.php (this file is included by kernel/content/edit.php, contains the PHP code to get the array you want, search for $groupedRelatedObjects)