The interface between Clients and Servers is defined as a set of Services. These Services are organized into logical groupings called Service Sets. Service Sets are discussed in 0 and specified in OPC 10000-4.
OPC UA Services provide two capabilities to Clients. They allow Clients to issue requests to Servers and receive responses from them. They also allow Clients to subscribe to Servers for Notifications. Notifications are used by the Server to report occurrences such as Alarms, data value changes, Events, and Program execution results.
OPC UA Messages may be encoded as text (XML or JSON) or in binary format for efficiency purposes. They may be transferred using multiple underlying transports, for example TCP or HTTP. Servers may provide different encodings and transports as defined by OPC 10000-6.