diff --git a/aip/general/0203.md b/aip/general/0203.md
index 4643c09d99..0043c25eba 100644
--- a/aip/general/0203.md
+++ b/aip/general/0203.md
@@ -127,6 +127,9 @@ A field **may** be described as optional if it is a field on a request message
 (a message that is an argument to an RPC, usually ending in `Request`), or a
 field on a submessage.
 
+**Note:** The field presence label `optional` and the field behavior
+`OPTIONAL` are not mutually exclusive. See AIP-149 for more details.
+
 ### Output only
 
 The use of `OUTPUT_ONLY` indicates that the field is provided in responses, but