(I know, 2-month old thread, but I just noticed this myself so thought I'd post for future reference if anyone else wonders):
I think it's this:
https://rogueamoeba.com/licensing/
"Audio Capture Engine (ACE)
Rogue Amoeba's “Audio Capture Engine” (abbreviated as ACE) is a MacOS-based technology which makes it possible to capture audio from one or more individual processes, as well as all audio from the entire system at once. ACE's functionality powers several of our most popular products.
Other software which might take advantage of ACE include screen sharing, VNC/remote access, game streaming, and more.
The software listed below uses ACE for audio capture:
Rogue Amoeba's own Airfoil for Mac, Audio Hijack, Loopback, and Piezo
Intel Unite® (Mac version)"