iOS 7 Inter-App Audio - the technical details we've been waiting for
This is an Apple developers document, "Audio Unit Framework Reference" which is way over my head. But it has all the official details of audio unit hosting which is the foundation of inter-app audio.
And thanks to Adil @rhism who gave a nice summary in another thread:
IAA introduces a new way for apps to send MIDI to each other ('new' meaning 'separate from CoreMIDI / Virtual MIDI'). An "AB input slot" app can be an IAA 'generator' node that just sends audio to its host, or it can be an 'instrument' node that receives MIDI from the host and sends audio based on that MIDI. So you can sequence the instrument node from the host.
https://developer.apple.com/library/ios/releasenotes/General/WhatsNewIniOS/Articles/iOS7.html (search for Inter-App Audio).
There are also 'effect' nodes that receive and send audio, and 'music effect' nodes that receive audio and MIDI, and send audio. Fun fact: the internal iOS code for an effect type node is 'aufx' :)