Hello, I'm a beginner / intermediate user of a Fireface UCX II.  I've spent hours trying to figure something out and am stuck.  I'm hoping someone can help me here.  I'm going to try to carefully explain my issue with requiring screenshots but happy to post screenshots later if this isn't clear.  I'm very close to getting what I want but one or two final things have me at a road block.  I'm pretty sure this is related to me not understanding how the FF works.

At a high level, I'm using Logic to send a backtrack to the FF.  The Audio Device is set to FF and I've chosen outputs 3/4.  In TotalMix (TM), I see the signal coming in on the Software Playback slider 3/4.  All is good.  Next I'm using Mimolive video streaming/recording software.  I set up an audio source in Mimo and chose the FF and Channels 3 and 4 for left and right, respectively.  At first it didn't work and I couldn't figure out why.  Then I learned about loopback.  I selected the Analog Output 3/4 at the bottom of TM and connected this with the Software Playback slider 3/4 and then turned on the loopback button.  And it worked!  I have a source in Mimo that is receiving a signal from a track in Logic going to output 3/4. 

Question #1:  But what is Output 3/4?  Is the FF sending a signal to my hardware output and I'm just not using it, but I need to use it to loop back the signal so that Mimo can receive it?  This is confusing.  If Logic is sending the backtrack to Output 3/4 and the FF is receiving that signal on 3/4, then why does Mimo not see 3/4 until I turn on loop back? 

I have monitors connected to the XLR output breakout cable.  But I don't understand which output number this represents.  There is a slider called Main that receiving all output to my speakers.  Is this Hardware Output 1/2?

The signal from Logic (Software Playback slider 3/4) is also going to the Main output which I need to hear the speakers.

Now here is where I get confused.  I want to use the FF for all other inputs.  I have a guitar processor that is connected to the SPDIF input of the FF.  This signal is seen coming in on a Hardware Input (top row of TM) called SPDIF.   It's being sent to the Main output and through my speakers also.  All is well.  But I also want to send this specific SPIDF guitar signal to Mimo.  I set up another source in Mimo that chooses the FF and Ch 1 and Ch 2 as left and right.  But I don't understand what is Ch1 and Ch2?  Is this where SPIDF sends the input signal from my guitar processor?  Why did I have to choose Ch1 and Ch2 to hear the guitar in Mimo.  I also had to select loopback on the Main slider to get it to work.   This is where the problem starts.

When I turn on the loopback on the Main output, then in Mimo the Logic track going to the FF is going to both Mimo audio sources.  In other words, the audio source I set up to only receive the guitar from the SPIDF on the FF, is also receiving the backtrack signal.   I want each audio source in Mimo to only receive a particular input on the FF (but I want to monitor both on my speakers connected to my FF).   The backtrack signal supposedly coming from 3/4 from Logic is coming through Mimo (set to ch3 and ch4), but it's also going to the audio source in Mimo connected to Ch1 and Ch2.  Why?

I think this has something to do with Ch 1 and Ch 2 that I don't understand. It seems to be combining all things looped back.

Here is what I want to do if above isn't clear.

I want to have the backtrack from Logic come into the FF (using outputs 3/4) and the guitar to come into the SPIDF (I have no idea what channel that is) and then have both of those signals monitored together through my speakers on the Fireface.  Then in Mimo I want to have two separate audio sources - one taking in the backtrack signal from the FF (ch 3/4 from logic) and the other taking in the guitar from the FF (coming from SPIDF input). 

For some reason when I use loopback on 3/4 and Main together it ends up sending the backtrack signal from 3/4 to both Mimo audio sources. 

Question:  What input channel does SPIDF represent? It's confusing that in Logic I can set the output to go to 3/4 and I see it on 3/4 slider in TM.  But when my guitar processor comes into SPIDF I have no idea what that means for channels.  I see it come through the Main and my speakers, and I chose CH1 and CH2 in Mimo to get th signal (and it works).  But that same source in Mimo (connected to CH1 and CH2) is picking up the backtrack from 3/4 also.  And if I turn loopback off on the Main slider, then I dont get the guitar signal in Mimo at all.

I hope this is clear.  I'm very confused between output numbers, names like "Main", speaker output mapping etc.  I tried to figure it out from the Matrix window but got further confused by the vertical and horizontal labeling.  I'm missing context and not able to visualize the paths in my head.  Where is it clear in Total Mix what signal is going out of the FF and into the computer and then into any apps receiving those signals?  How do I know exactly what outputs channel numbers are going from the FF, through the USB, to my computer and then to other apps?  How do I make sure they are all separated and won't be combined by other apps if I want the signals all separate. It seems many apps can't see the signal without Loopback on, but then it sometimes combines signals when I don't want it to.

I would greatly appreciate some guidance here.  Thank you!!!