Adding User Defined Interface Data (UDID) to Sabre PNR

I'm trying to add UDID fields to PNRs using the SOAP API. There is no mention of such functionality anywhere in the docs!

So I tried using the SabreCommandLLSRQ endpoint to attach the UD fields, but I don't seem to be able to make any changes, despite receiving a * response.

I've tried the following command using the endpoint:

5.S*RL[record locator]*UD56 [some test value]

For example:

5.S*RLEPLHYN*UD56 YVRYEG

But that also leads to a * response without the PNR actually changing.

Are you sure that's the right UDID format? I've never seen a record locator prepended to a UDID before. If using the SabreCommandLLSRQ endpoint I would expect your format to look like this:

5.S*UD56 [some test value]

There are many UDID formats so I may be wrong there and I know the responses can be quirky. What response do you get in a PNR if you simply enter your format into Sabre Red?

There is also support for itin remarks in the Add Itinerary Remark (AddRemarkLLSRQ), which may be worth implementing for you as well.

I found it in the API too, it is only mentioned in one of many examples in AddRemarkRQ:
If you want it to show up on the itinerary:

<AddRemarkRQ xmlns="http://webservices.sabre.com/sabreXML/2011/10" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" Version="2.1.1">
	<RemarkInfo>
		<Remark Type="Invoice">
			<Text>
				U45-111794
			</Text>
		</Remark>
	</RemarkInfo>
</AddRemarkRQ>

If you don't want it to show up on the itinerary:

<AddRemarkRQ xmlns="http://webservices.sabre.com/sabreXML/2011/10" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" Version="2.1.1">
	<RemarkInfo>
		<Remark Type="Invoice">
			<Text>
				U*45-111794
			</Text>
		</Remark>
	</RemarkInfo>
</AddRemarkRQ>

I've tested it in both CreatePassengerNameRecordRQ and AddRemarkRQ contexts.