The Client Application is the code that implements the function of the Client. It uses the Client API to send and receive OPC UA Service requests and responses to the Server. The Services defined for OPC UA are described in 7, and specified in OPC 10000-4.
Note that the “Client API” is an internal interface that isolates the Client application code from an OPC UA Communication Stack. The OPC UA Communication Stack converts Client API calls into Messages and sends them through the underlying communications entity to the Server at the request of the Client application. The OPC UA Communication Stack also receives response and NotificationMessages from the underlying communications entity and delivers them to the Client application through the Client API.