Data importer behavior when importing data into an existing record is controlled using the existing_record_policy option in data importer mappings. When set to merge, values are added to existing values in the record. When set to a _with_replace policy, values will overwrite existing ones.
The existing record policy applies the entire mappings. Occasionally it's useful to be able to mix add and replace behavior within a mapping. The new "add" and "replace" importer options force the relevant mapping to behave in the specified manner. These options must be set for each mapping line for which behavior is to be specified.