diff --git a/dom/bindings/Bindings.conf b/dom/bindings/Bindings.conf
index ed73bd85838c..580b0b54c118 100644
--- a/dom/bindings/Bindings.conf
+++ b/dom/bindings/Bindings.conf
@@ -21,7 +21,8 @@
 #   * concrete - Indicates whether there exist JS objects with this interface as
 #                their primary interface (and hence whose prototype is this
 #                interface's prototype object).  Always False for callback
-#                interfaces.  Defaults to True otherwise.
+#                interfaces.  Defaults to True for leaf interfaces and
+#                interfaces with constructors, false otherwise.
 #   * notflattened - The native type does not have nsIClassInfo, so when
 #                    wrapping it the right IID needs to be passed in.
 #                    Only relevant for callback interfaces.