diff --git a/app/config/locale/countries.php b/app/config/locale/countries.php
index 33673cdc9b..07a418da9c 100644
--- a/app/config/locale/countries.php
+++ b/app/config/locale/countries.php
@@ -1,5 +1,13 @@
 <?php
 
+/**
+ * ISO 3166 standard country codes
+ * https://www.iso.org/iso-3166-country-codes.html
+ *
+ * Source:
+ * https://www.iso.org/obp/ui/#search/code/
+ */
+
 return [
     'AF',
     'AO',