This data item is specified which element data items should be taken for the element index. Example: elements 0 and 1 get density 1024 while element 2 gets density 1236
element 0 0 1 2
element 1 1 2 3
element 2 2 3 4
...
element_group 0 1
element_group 1 1
element_group 2 2
...
density 1 1024.
density 2 1236.
If no element_group records are specified, all element data should use index is 0.
See also area_element_group.