The value of this data item is the identifier of the
               set of axes for which axis settings are being specified.
               Multiple axes may be specified for the same value of
               _array_structure_list_axis.axis_set_id.
               This item is a pointer to
               _array_structure_list.axis_set_id
               in the ARRAY_STRUCTURE_LIST  category.
               If this item is not specified, it defaults to the corresponding
               axis identifier.