diff --git a/index.bs b/index.bs
index 6bab262..c59b118 100644
--- a/index.bs
+++ b/index.bs
@@ -1514,7 +1514,12 @@ Appendix A: Messages {#appendix-a}
 
 The following messages are defined with [[CDDL]]. When
 integer keys are used, a comment is appended to the line to indicate
-the name of the field. Each root message (one that can be put into a
+the name of the field. This unusual syntax is consistently applied for object
+definitions in this specification due to it resulting in significantly smaller
+objects over-the-wire. Integer keys are used instead of object arrays to allow
+for easy indexing of optional fields.
+
+Each root message (one that can be put into a
 QUIC stream without being enclosed by another message) has a comment
 indicating the message type key.