diff --git a/MessagingApp.markdown b/MessagingApp.markdown
index fc28f50..f620808 100644
--- a/MessagingApp.markdown
+++ b/MessagingApp.markdown
@@ -18,9 +18,9 @@ MessagingApp uses a single channel with a random identifier:
 
 ### Storage policy
 
-* All local messages are stored.
-* All remote messages are stored.
-* Messages of type ATTACHMENT that are not referenced are garbage-collected.
+* All valid local messages are stored.
+* All valid remote messages are stored.
+* Messages of type ATTACHMENT that are not referenced by messages of type PRIVATE_MESSAGE are garbage-collected.
 
 ### Sharing policy