In TotalMix FX
1. Ensure you are in submix mode  (right side under "Routing Mode" blue field)
2. In the Bottom Row "Hardware Outputs" click to the Phones Output
    Now you see the submix for the phones output by watching to the fader positions of:
   a) HW Inputs (top row), this is the audio signal arriving at the input of your recording interface with near zero-latency
   b) SW Playback Channels (middle row), this is the audio signal which comes from your PC/application
3. Create your submix, so .. if you want to hear more from:
    - the Mic input: then adjust this audio level by raising the fader of the particular input(s) in the top row
    - DAW/PC:        then adjust this audio level by raising the fader of SW playback channels in the middle row

With a very compact and small recording interface which supports only very few channels such a "blend/Mix" knob might be feasible, but not if you have many more channels and want to give the user more flexibility.

Compare it with analog mixers. Only the better ones offered subgroups to allow for individual submixes. But due to the engineering efforts and simply the resulting size of such the mixer, there were limits. Most that I remember had not more than 4, 8 or 16 submixes. Maybe there were also bigger pults available ...

In contrast to that TM FX gives you as many subgroups / submixes as you have HW Outputs on your Recording Interface.
For some of the bigger cards up to 198 output channels / submixes.

The routing is quite simple once you understood this basic principl
A) submix per HW output and
B) to create / modify a submix:
- click to a HW Output aka "Submix"
- move faders of HW inputs (from interface) and SW playback channels (from PC/Application/DAW) as needed