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.