Inter-App Audio
Apple's first mechanism for passing audio and MIDI between music apps on iOS, introduced in iOS 7 and superseded by AUv3.
Early iPad music apps were islands: a synth couldn't record into another app without detouring through the clipboard or files. The first fix didn't come from Apple but from Audiobus, a standalone app launched in December 2012 that patched virtual cables between apps that supported it. Apple responded in 2013 with Inter-App Audio in iOS 7, letting a host such as GarageBand launch another app in the background and receive its audio directly.
It shared Audiobus's weakness: every source was a whole app, with its own UI and its own memory budget. Once AUv3 arrived in 2015, effects could instead be loaded as extensions inside the host. Apple deprecated Inter-App Audio in iOS 13 in 2019, and apps built on it had to choose between a rewrite and silence.