At the start of the calculations, this record is used to generate element_group records. Each element, all of whose nodes are part of the geometry_item, will get an element_group record with value element_group. Please realise that the geometry entity can be a two-dimensional area, a volume, etc.
This option comes handy whenever a part of the domain gets some specific element data. By example, this would be the case if different areas in the structure have different material properties like stiffness, etc.
Beware: any directly specified element_group records will be overwritten.