OPC UA specification provides a standard Information Model domain, which contains a set of predefined types and instances. This domain may be extended by designing a new one. Even though the standard Information Model contains a rich set of predefined types, the type concept allows designers to freely define types according to the application needs. New types are derived from the existing ones. The derived types inherit all features of the base types but can include modifications to make the new types more appropriate for information to be represented. This new information model covered by the domain may be the subject of a companion specification or proprietary solution. In any case new definitions must be uniquely named and self-contained except for external type references. All not predefined types (not belonging to the standard domain) must be exposed in the Address Space by the server.