Difference between displayed_column and displayed_linked_columns

What is difference in column settings strings displayed_column and displayed_linked_columns?

            "id": "subitems_total",
            "type": "mirror",
            "title": "Subitems Total",
            "settings_str": "{\"relation_column\":{\"subitems\":true},\"displayed_column\":{\"numbers\":true},\"displayed_linked_columns\":{\"5059360432\":[\"numbers\"]}}"

I see we get columnId of subboard column that is summary column on main board, Is displayed_column specifically for summary of subItem board column? Also it seems like we can also relate to it using displayed_linked_columns which seems to be generic for all mirror columns?