The interface between Clientsand Serversis defined as a set of Services.These Servicesare organized into logical groupings called Service Sets. Service Setsare discussed in 0and specified in OPC 10000-4.

OPC UA Servicesprovide two capabilities to Clients. They allow Clientsto issue requests to Serversand receive responses from them. They also allow Clientsto subscribe to Serversfor Notifications. Notificationsare used by the Serverto report occurrences such as Alarms, data value changes, Events, and Programexecution results.

OPC UA Messagesmay 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. Serversmay provide different encodings and transports as defined by OPC 10000-6.