diff --git a/autoware_adapi_v1_msgs/system/msg/Heartbeat.msg b/autoware_adapi_v1_msgs/system/msg/Heartbeat.msg
index f9b50bf..1b20842 100644
--- a/autoware_adapi_v1_msgs/system/msg/Heartbeat.msg
+++ b/autoware_adapi_v1_msgs/system/msg/Heartbeat.msg
@@ -1,2 +1,4 @@
 builtin_interfaces/Time stamp
+
+# Sequence number for order verification, wraps at 65535.
 uint16 seq