"TotalMix FX" is simply the product name of the newer TotalMix version.
The letters "FX" have nothing to do whether there are FX in a recording device or not.
So you have the same routing possibilities in Digiface USB like everybody else.

There are sometimes minor exceptions based on the HW, which are then documented in the manual. But routing audio through an HW output in submix mode and recording the wet signal from a HW input is a basic routing functionality that all recording interfaces have.

You have two possibilities:

a) routing inside TM FX

Send audio to an output which is connected to the input of an external effect.
Connect it's output to another input and then you can record the wet signal there at the analog input.

b) define external device in the DAW if it is supported

If you have a DAW like Cubase then it's even simpler, there you can define external FX devices by configuring an output and input port and then you can use it like a VST. Cubase then also automatically "pings" through the device to calculate the RTL properly.

Regarding Option a) and routing in TM FX

- Best being performed in TM FX "submix mode" (check TM FX upper right, blue area).
- If you want to route audio to an output, then you select this Hardware Output in the bottom row of TM FX.
- Then you can create an individual submix for this particular output by raising the faders of
   - HW inputs (top row with very low near-realtime latency) or
   - SW playbacks (middle row with the typical RTL through USB/TB)
- If your signal comes from
  - an input, raise the fader of this HW input in the top row ....
  - the PC / the DAW, raise the fader of this SW playback in the middle row ....
                                                                                                  .... and you'll see a signal arriving at the ext. HW.

My recommandation to you

Try a DAW like Cubase, the handing there with external devices is meanwhile extremely easy.
I personally would prefer b) when working with a DAW because of
- the very easy handling and
- the automatic RTL calculation that takes place

See this nice video from Greg Ondo: https://www.youtube.com/watch?v=H-N-9AGlrXk
This video shows how easy it is to integrate external FX into cubase.

Most important, the wet audio will return in time, as Cubase can perform a delay calculation here.
This you wouldn't have when only performing the routing through TM FX.

For a great overview about routing in TM FX, see this video:
https://www.youtube.com/watch?v=CcWClPViBm0

Here I compiled a list of different RME training videos:
https://www.tonstudio-forum.de/blog/ind … al-Videos/

ok thanks understood now.