+ /**
+ * The sequence number of this update. Updates *must* be replayed in-order according to this
+ * sequence number (and updates may panic if they are not). The update_id values are strictly
+ * increasing and increase by one for each new update, with one exception specified below.
+ *
+ * This sequence number is also used to track up to which points updates which returned
+ * ChannelMonitorUpdateErr::TemporaryFailure have been applied to all copies of a given
+ * ChannelMonitor when ChannelManager::channel_monitor_updated is called.
+ *
+ * The only instance where update_id values are not strictly increasing is the case where we
+ * allow post-force-close updates with a special update ID of [`CLOSED_CHANNEL_UPDATE_ID`]. See
+ * its docs for more details.
+ *
+ * [`CLOSED_CHANNEL_UPDATE_ID`]: constant.CLOSED_CHANNEL_UPDATE_ID.html
+ */