Encoding of data type fields which use abstract data types|OPC UA Standard|Forum|OPC Foundation

Good day everybody!

What is the proper (binary) encoding of a data type field which uses an abstract data type?

Example from BACnet:

<UADataType NodeId="ns=1;i=103010" BrowseName="1:BACnetTimeValueValue">
  <Definition Name="1:BACnetTimeValueValue">
    <Field Name="BooleanValue" DataType="Boolean" />
    <Field Name="UnsignedValue" DataType="UInteger" />
    <Field Name="SignedValue" DataType="Integer" />
    ...
  </Definition>
</UADataType>

What is the proper encoding for the fields "UnsignedValue" and "SignedValue"? Based on the following statement in OPC UA v1.05 Part 6 5.1.7 Variant, I suspect that a variant is used for proper encoding:

Variantsare used to store any value or parameter with a data type of BaseDataType or one of its subtypes.

Is there another passage in the specification that correctly describes the (binary) encoding of abstract data types?

Thanks in advance!

Greetings,

Sebastian Schwinn