An ExtensionObject is a container for any Structured DataTypes which cannot be encoded as one of the other built-in data types. The ExtensionObject contains a complex value serialized as a sequence of bytes or as an XML element. It also contains an identifier which indicates what data it contains and how it is encoded.

Structured DataTypes are represented in a Server address space as sub-types of the Structure DataType. The DataEncodings available for any given Structured DataTypes are represented as a DataTypeEncoding Object in the Server AddressSpace. The NodeId for the DataTypeEncoding Object is the identifier stored in the ExtensionObject. OPC 10000-3 describes how DataTypeEncoding Nodes are related to other Nodes of the AddressSpace.

Server implementers should use namespace qualified numeric NodeIds for any DataTypeEncoding Objects they define. This will minimize the overhead introduced by packing Structured DataType values into an ExtensionObject.

ExtensionObjects and Variants allow unlimited nesting which could result in stack overflow errors even if the message size is less than the maximum allowed. Decoders shall support at least 100 nesting levels. Decoders shall report an error if the number of nesting levels exceeds what it supports.