Excluding is just splitting without doing something with the excluded part :).
Say everything on your PC goes to your default audio. If you had two soundcards you could have Skype output to the second one. Then in OBS MP you make two Audio-output sources. One for each soundcard. You can then assign those to streams separately.
But since we don't HAVE multiple audio cards we fake it by making a virtual one. In this scenario Skype outputs to the virtual audio cable. You can then create an audio-output source to record off of that output.
Alternatively, since the virtual cable has two ends, you can make an audio-input source instead and have OBS MP treat it as just another microphone.
What I do is a bit different with Voicemeeter (not Voicemeter :) ) is that I'll make ANOTHER virtual cable and use it as my Default PC output. Then I'll have Voicemeeter record from all those cables and output to my headset as well as a third cable. In Voicemeeter you can then adjust volume levels and select which of its outputs (My headset and Cable#3) gets to receive its inputs (Cable#1, Cable#2, my Microphone...). Cable #3 then goes to OBS as a "fake" microphone.
That way my headset gets my PC Audio and Skype while OBS gets PC Audio, my microphone and Skype if I want it to. If I want, with the click of a button, I can enable my microphone for my headset, check audio levels, adjust the mix and disable it again without interrupting the OBS Audio.
The pre-mixing in Voicemeeter is for streaming, but on top of that, to have multiple audio tracks in recording I'll do the following:
In OBS MP I'll add the separate sources as well, BUT I assign them to different Audio Streams (the streaming engine listens to Audio Stream 1 and I set the recording engine to record all four Audio Streams).
- Audio Stream 1: Voicemeeter's pre-mixed audio (Cable#3)
- Audio Stream 2: PC Audio (Cable#2)
- Audio Stream 3: Microphone
- Audio Stream 4: Skype (Cable#1)
The reason I also record the pre-mixed audio is because, when you play the video file, it by default only plays track 1 which, in this case, has everything (possibly excluding Skype).