Doesn’t “Aux” usually refer to audio? I think what you’re looking at is an audio output that you could plug headphones (or any 3.5mm device) into to hear the audio; it’s not a video signal.

It might help if we knew exactly what device you’re looking at in this screenshot. If that’s a capture card, then that is exactly what I described - the capture card is outputting the audio so you can plug in headphones to hear it. You see this with some cards as a way to prevent you from needing to run the audio channels through a mixer - you can plug your headphones into the aux out to hear the game audio without needing to mix the audio signal on your computer.