diff --git modules/field_ui/field_ui.api.php modules/field_ui/field_ui.api.php
index c8e0991..b9a9623 100644
--- modules/field_ui/field_ui.api.php
+++ modules/field_ui/field_ui.api.php
@@ -173,6 +173,10 @@ function hook_field_formatter_settings_form($field, $instance, $view_mode, $form
 /**
  * Returns a short summary for the current formatter settings of an instance.
  *
+ * If an empty result is returned, the formatter is asuumed to have no
+ * configurable settings, and no UI will be provided to display a settings
+ * form.
+ *
  * @param $field
  *   The field structure.
  * @param $instance