Hint indicating to the user agent whether the application, upon calling getDisplayMedia() with {systemAudio: true}, wishes *system audio* to be offered to the user. (If not - only offer tab-audio.)
User agents may offer audio to be captured alongside video, if the application specifies {systemAudio: true}, or maps audio to anything else that's different from false. But not all audio is created alike. Consider video-conferencing applications. Tab-audio is often useful, and can be shared with remote participants. But system-audio includes participants' own audio, and it is NOT desirable to share it back. State of the art? VC applications can ask for "audio", use it if it's tab-audio, and discard it otherwise. This works, but it's sub-optimal. The user is left confused. The user wanted to share system-audio, the user was offered to share-system, the user explicitly approved sharing system-audio - and now remote participants are telling the user that they can't, in fact, hear the system-audio. Now how confusing is that?! It’s better to allow applications to ask for less - allow the application to ask for non-system audio.
Explainers: https://docs.google.com/document/d/1q3oGy7hLJmdQA4ZK7QG7DnwgtcpL6oB2pqLQJ6MP1tY/edit?usp=sharing