diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index aba4595..61350a9 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -170,6 +170,8 @@ Each User ID carries associated preference settings, such as preferred encryptio
 
 i think crypto-refresh suggests that the direct key signature should hold the default preferences?
 we might need to write a more nuanced text here, about how DKS and primary user id interact in v6, and mention the differences to v4? 
+
+the primary User ID can also specify metadata about the primary key
 ```
 
 ### User attributes in OpenPGP