IEC 62541-4:2011 defines the OPC Unified Architecture (OPC UA) Services. The Services described are the collection of abstract Remote Procedure Calls (RPC) that are implemented by OPC UA Servers and called by OPC UA Clients. All interactions between OPC UA Clients and Servers occur via these Services. The defined Services are considered abstract because no particular RPC mechanism for implementation is defined in this part of IEC 62541. IEC 62541-6 specifies one or more concrete mappings supported for implementation. For example, one mapping in IEC 62541-6 is to XML Web Services. In that case the Services described in this part of IEC 62541 appear as the Web service methods in the WSDL contract.