This item is a pointer to _array_structure_list.index
              in the ARRAY_STRUCTURE_LIST  category.
              Identifies the one-based index of the row, column, sheet ...
              the ARRAY_STRUCTURE_LIST  category.
              For a multidimensional array, a value must be explicitly given.
              If an index is omitted from a section then all elements for that
              index are assumed to be included in the section.