This rollout lets you check selections to see if any are overlapping; that is, if a material ID or a face or element is assigned to more than one selection.
After you click Check, the fields in this group display the results. The first field is for material IDs, and the second is for sub-object selections. If there is no conflict, the first field says “No conflicting Mat IDs detected,” and the second says “No conflicting face selections detected.” If there is a conflict, the results say something such as, “6 Mat IDs are assigned to more than one selection.”