diff --git a/docs/source-2.0/aws/aws-auth.rst b/docs/source-2.0/aws/aws-auth.rst
index f07967b979e..4bfaf6db7ff 100644
--- a/docs/source-2.0/aws/aws-auth.rst
+++ b/docs/source-2.0/aws/aws-auth.rst
@@ -6,6 +6,12 @@ AWS Authentication Traits
 
 This document defines AWS authentication schemes.
 
+If a request contains the ``Authorization`` header or a query string parameter
+with the name of ``X-Amz-Algorithm`` containing the value ``AWS4-HMAC-SHA256``,
+the request will undergo authentication and be rejected if it fails. Otherwise,
+if the :ref:`optionalAuth-trait` is applied, the service shall operate on the
+unauthenticated request.
+
 .. smithy-trait:: aws.auth#sigv4
 .. _aws.auth#sigv4-trait: