def METHOD_NAME(self) -> dict:
    """Returns a nested dictionary which describes the mapping to the data field.
    The dictionary is of the form {"mod_key": mod_key, "attr_name": attr_name, "attr_key": attr_key}.
    This mapping is then combined with the mappings of other fields to make up the data registry.
    """
    if self.is_empty:
        return {}
    data_registry = {
        _constants._DR_ATTR_NAME: self.attr_name,
        _constants._DR_ATTR_KEY: self.attr_key,
    }
    if self.mod_key is not None:
        data_registry[_constants._DR_MOD_KEY] = self.mod_key
    return data_registry