It's a generic source. Could be anything.
I use a Mic source, connected to a USB line input, that is fed from a physical console. OBS sends that both to the Output, which goes to the stream and recording, and to the Monitor, which goes to my headphones, so I can mix a band on that console to what OBS is actually getting.
Actually, since all of my audio processing is done in the console, so its finished soundtrack is my *only* live input to OBS, I can afford to connect 3 global "mic" sources to that same line input. One goes to the Output only, in stereo, one goes to the Monitor only, in stereo, and one goes to the Monitor only, in mono. The mono'ing is done in OBS, using the checkbox in the Advanced Audio Properties. I have a pair of hotkeys set up to mute/unmute the two Monitor-only copies, so I effectively have a stereo/mono switch on the keyboard, without affecting the stream. That lets me check the mix both ways and make sure it still works, since a lot of viewers have their speakers too close together to appreciate a stereo image.
And I've been PM'ing a guy on here for quite a while now, about his playing a guitar and singing, while recording or streaming that in OBS. What (good and cheap) gear to buy, how to use it, how to set up OBS, pretty soon figured out that he really needed something better for audio processing and then bring that into OBS as a finished soundtrack, etc.
Even outside of music, the Monitor is still useful for a mic input, even a real mic. Because it's the only audio out of OBS that isn't tied up in a stream or recording, that's how you get it into a remote meeting like Zoom, Skype, Jitsi, etc. OBS operates as if it were running a live stream, except that it's not actually streaming. Instead, the meeting picks up OBS's Virtual Camera, just like any other camera, and the audio comes from OBS's Monitor, through a loopback. A loopback is a virtual speaker and a virtual mic, so that whatever you send to that "speaker" goes nowhere physical but appears in that "mic". Set up the apps accordingly; they don't know the difference between that and a physical mic or speaker.
Of course, the ability to send a mic to a speaker *does* create the possibility of a feedback loop. But to remove that ability would break a TON of different workflows. If it's going to be flexible enough for anyone to use, then you kinda need to understand how to run a console, even if you don't have one, to use it well. Because the same pitfalls exist for the same reasons.