diff --git a/msg/vehicle_command.msg b/msg/vehicle_command.msg
index 9403bc9ea9..f5803533ae 100644
--- a/msg/vehicle_command.msg
+++ b/msg/vehicle_command.msg
@@ -155,6 +155,7 @@ uint8 FAILURE_TYPE_SLOW = 5
 uint8 FAILURE_TYPE_DELAYED = 6
 uint8 FAILURE_TYPE_INTERMITTENT = 7
 
+# used as param1 in DO_CHANGE_SPEED command
 uint8 SPEED_TYPE_AIRSPEED = 0
 uint8 SPEED_TYPE_GROUNDSPEED = 1
 uint8 SPEED_TYPE_CLIMB_SPEED = 2