A DC Instrument is for display stereo channels and you can select those...
Not clear what your question is.
After your question you write it works well, so what is the question ?

I do not understand the question.... What do you mean by 'Instrument'?

And do you speak about DigiCheck or DigiCheck NG?  DG NG is the newer one.

Playback in Digiface USB has just ADAT 1-32 and Phones, no other "outputs".

Sorry, I do not get your point as well.

On the one hand you talk about channels and then you talk about instruments which is the new name for all the DIGIcheck tools.

Lets start with DIGIcheck NG and instruments. You need to select first, how many instruments you intend to use in an DIGIcheck NG instance.

Then for every analysis tool ("instrument") you need to select the channels, either input or playback channels.
If you want to measure outputs, then you need to select Loopback.

At least for me as Windows user this is the case, also with the old DIGIcheck, that I can only select HW input and SW playbacks.

Well and if your Digicheck USB has only 32 ADAT and 2 ANALOG channels (phones) then you will see only
32 HW inputs and
32+2 SW Playback channels

And I fear for the two phones outputs there is no possibility for loopback as a corresponding input is missing.
You should see, whether this output offers a loopback button, most likely not.

Should I have misinterpreted your question, then please rephrase in an understandable way and please do not mix the terms "instruments" and "channels".

You don't need to use Loopback you can choose Adat 1 to 32 every channel.
You may have to know wich channel you did assign to main-out.
Then select that number.
Digicheck does not read out Totalmix, it can't know wich channel you did assign to Main-out.
In Totalmix control room bottom right side the big button ASSIGN, click there to find out what is assigned.

Ah I see. Yes you are right, you still have to use a Loopback to monitor hardware output channels.

Digicheck directly only access input channels and playback - at moment

If RME will upgrade this functionality ? Who knows...

I would say, yes, please, it would be a useful feature.

I suspect that there is a good reason for this, otherwise RME would have already implemented it. At the latest, with the new implementation of DIGICheck (DIGICheck NG for Windows and Apple) it would have been possible to incorporate this now.

This probably has to do with how DIGICheck accesses or can only access the data.

I assume it's because the audio data from HW inputs and SW playbacks are directly accessible by the PC by reading audio data from HW inputs and writing data to the SW playback channels.

This is not the case with the HW outputs, as you can also route audio data from inputs in "near real-time" directly on the recording interface without increased latency (RTL) over the PC.

Perhaps this makes the difference that data of the HW outputs must first be sent back to the inputs via loopback to be "complete". Otherwise, you might get only SW playback data and not the direct routing of audio data from HW inputs to HW outputs, which happens directly on the recording interface.

Does this also make sense for you?

Perhaps RME can briefly explain/clarify why this is not possible, as there is interest in such a functionality for DIGICheck to directly analyze data from HW outputs without using loopback. Many thanks.

The Mac DriverKit driver allows direct playback reading.