Even the typical refernce "HasEncoding" is missing at this data type.
So there is no chance to handle this data type as other structures.

Is it intended, that this data type has to be implemented like a built-in type?
If yes, that please state this explicitely in specification.

Even built-in types (e.g. "LocalizedText") are described in dictionary, so why not this data type?

I think the DecimalDataType is something specific to the Foundation's .NET implementation. There are a lot of other types that do not exist as well. However it should be noted that this is only in the "Services" NodeSet, as far as I'm aware the current 1.04.7 standard nodeset does not have this. In addition at least now it is propertly marked as "CodeGenerator" specific, which to my knowledge means it is not a real type, only something the .NET impl needs.

In our Prosys OPC UA SDK for Java codegen I'm ignoring every CodeGenerator marked type as they are not real types

(in the services nodeset it is like this at the moment):
<UADataType NodeId="i=17861" BrowseName="DecimalDataType" Purpose="CodeGenerator">
<DisplayName>DecimalDataType</DisplayName>
<References>
<Reference ReferenceType="HasSubtype" IsForward="false">i=22</Reference>
</References>
<Definition Name="DecimalDataType">
<Field Name="Scale" DataType="i=4" />
<Field Name="Value" DataType="i=15" />
</Definition>
</UADataType>

And I think the original spec text of "like other structures" mean as a Structure for which there is no encoding defined, i.e. it will remain as binary/xml/json raw ExtensionObject. So basically it is a "built-in" type without taking a built in type number (of which are limited amount spaces left).

Here is a quote from an email I received about this:

We are currently on the way of defining the requirement for the decimal data type to be implemented in the SDK. On this way we are struggling a little bit. The information regarding Decimal is quite confusing in specification.

It is represented below “Number” and has

no data type description assigned
no information in data type dictionary
not even a “HasEncoding” reference
In the binary dictionary a data type “DecimalDataType" is described, which looks “a little bit” like the “Decimal” data type, but is not identical to the encoding described in Part 6.

So we are not sure how we have to

implement encoding / decoding
represent this data type in our SDK

All this implies that we cannot treat the “Decimal” data type as any other structured type.

Add text to clarify in Part 6 that stacks that want to encode/decode this type need to custom code this like is done for all other built-in types because The "structure" metadata is not present in the address space/nodeset.

Fixed in Draft 22.

Agreed to changes in Telecon.