Sets the structure fields. The values of structure members are embedded recursively within the referencing field.
For example, if we have an employee class, which has a reference to a structure phoneNumber and the phoneNumber has two fields: number and areaCode. Any changes to the metadata properties for any structure field will be nested within this extension:
<extension key="field" value="areaCode">
<extension key="db-column-name" value="phone_number_area_code1" />