diff --git a/includes/form.inc b/includes/form.inc
index 939dd5a..b69fbb4 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -1436,6 +1436,10 @@ function _form_validate(&$elements, &$form_state, $form_id = NULL) {
       $is_empty_value = ($elements['#value'] === 0);
       if ($is_empty_multiple || $is_empty_string || $is_empty_value) {
 
+        // Flag this element as #required_but_empty to allow #element_validate
+        // handlers to set a custom required error message, but without having
+        // to re-implement the complex logic to figure out whether the field
+        // value is empty.
         $elements['#required_but_empty'] = TRUE;
 
         if (!empty($elements['#required_error'])) {