The set of Objects and related information that the Server makes available to Clients is referred to as its AddressSpace. The OPC UA AddressSpace represents its contents as a set of Nodes connected by References.
Primitive characteristics of Nodes are described by OPC-defined Attributes. Attributes are the only elements of a Server that have data values. Data types that define attribute values may be simple or complex.
To promote interoperability of Clients and Servers, the OPC UA AddressSpace is structured hierarchically with the top levels the same for all Servers. Although Nodes in the AddressSpace are typically accessible via the hierarchy, they may have References to each other, allowing the AddressSpace to represent an interrelated network of Nodes. The model of the AddressSpace is defined in OPC 10000-3.