The ClientApplication is the code that implements the function of the Client. It uses the ClientAPI to send and receive OPC UA Servicerequests and responses to the Server. The Servicesdefined for OPC UA are described in 7, and specified in OPC 10000-4.
Note that the “ClientAPI” is an internal interface that isolates the Clientapplication code from an OPC UA Communication Stack. The OPC UA Communication Stack converts ClientAPI calls into Messagesand sends them through the underlying communications entity to the Serverat the request of the Clientapplication. The OPC UA Communication Stack also receives response and NotificationMessagesfrom the underlying communications entity and delivers them to the Clientapplication through the ClientAPI.