The DataType TypeDefinitionAttribute can hold a StructureDefinition. The StructureDefinition has fields which are of a defined type. The type is required in order to facilitate encode/decode of the structure.

Is it a valid use case for an implementer to define a structure field as an abstract type such as BaseDataType or Structure? This would require using a variant or extension object on the wire at the structure field position in order to encode the field.

I would have thought that this was invalid. Incidentally, Part 6 states "ExtensionObjects are used in two contexts: as values contained in Variant structures or as parameters in OPC UA Messages."