iaa_features

IAA Details

Generator apps create sounds.
Instrument apps can have MIDI sent to their IAA MIDI port to control them.
Effect apps receive an audio signal and effect it in some way.
MIDI Effect an effect app that can be controlled via MIDI sent to its MIDI port.
An IAA app may listen for MIDI or have a virtual port even though it's only an IAA app (this can be very confusing). You may have setups where there can be duplicate MIDI messages being sent to the same IAA app which creates problems or you may need a third app to receive MIDI and send it on to the final destination.

An IAA host app must have implemented host sync and the IAA app must have host sync to sync up to the host's transport controls. This sync is sample accurate.

bug that occurs when you have IAA apps hosted in an IAA app host, you close the host, and the IAA app continues to run in the background. Even when you check to see if the app is open, it may not appear. The app running in the background will continue to use resources on your iOS device and can cause your iOS music setups to crash. Bringing the IAA app to the foreground and then closing it can help to minimize IAA zombies. Opening and closing the app in the setup can resolve the issue too.

At Apple’s WWDC 2019 they revealed that IAA has been deprecated. IAA apps will continue to work in iOS 13 and below, but at some point when Apple releases a major release of iOS and/or iPadOS they’ll stop supporting IAA and IAA apps will no longer work on those OSes.
Here’s an interview on CDM of Michael Tyson the Audiobus developer.