Source

File: wp-includes/rest-api/endpoints/class-wp-rest-controller.php

protected function get_additional_fields( $object_type = null ) {

		if ( ! $object_type ) {
			$object_type = $this->get_object_type();
		}

		if ( ! $object_type ) {
			return array();
		}

		global $wp_rest_additional_fields;

		if ( ! $wp_rest_additional_fields || ! isset( $wp_rest_additional_fields[ $object_type ] ) ) {
			return array();
		}

		return $wp_rest_additional_fields[ $object_type ];
	}