The field mapping features determine the representation of properties in the underlying relational database system. Persistent properties are mapped with one of the field mappings that store the property values in a database field. Field mappings are applied on abstract properties of persistent classes. Persistent properties are similar to the fields (member variables) of .NET classes.
The <NearObjectReference> element can be used to store references to the same DataDomain. In this case, the identity fields of the referred object are stored in one or more database fields. This concept corresponds to the foreign key paradigm of database systems.