UDTs in the Siemens program are exposed in the address space as both a collection of nodes that can be read, written or subscribed to individually as well as a single complex DataType accessible in the value attribute of the parent of the collection of nodes in the address space that represent the UDT.
Currently, Ignition does not support accessing these complex DataType values. If implemented, the PLC Data Types can be reused as Ignition UDTs and linked with a single OPC-UA address.