diff --git a/design.org b/design.org
index 3089fc0..36b426d 100644
--- a/design.org
+++ b/design.org
@@ -1,4 +1,5 @@
 # Copyright (C) 2018 Jan Malakhovski
+# Copyright (C) 2018 Leo Gaspard
 # Permission is granted to copy, distribute and/or modify this document
 # under the terms of the GNU Free Documentation License, Version 1.3
 # or any later version published by the Free Software Foundation;
@@ -164,6 +165,8 @@ of other reviewers
 
 "priority": self-explanatory.
 
+Unknown metadata fields with names starting with `x-` MAY be ignored or processed in an implementation-dependent way, while other unknown metadata fields SHOULD trigger a warning and MUST cause the review to be ignored.
+
 ** Examples
 
 "Nothing looks bad, but don't trust me" would be