au_features

AU / AUv3 / Audio Unit

AUv3 apps can be hosted inside AUv3 host apps provided the AUv3 host app supports its features. Some AUv3 hosts do not support AUv3 MIDI effect apps for example. You can run mutliple instances of AUv3 apps in a host or multiple hosts provided your iOS device has sufficient memory to support the apps running at the time and the total memory used by all AUv3 apps currently running.
AUv3 apps can not be hosted inside another AUv3 app.

AUv3 state saving. The state of an AUv3 host will be saved by the AUv3 host a part of the AUv3 host's project or setup.
AUv3 presets saved in the AUv3 app are available via the AUv3 app in whatever AUv3 host you use.
AUv3 presets saved to the AUv3 host app are only available to that specific host.
Some AUv3 apps do not support saving presets inside the AUv3 app itself.
Some AUv3 host apps do not support the ability to save presets to the host.

AUv3 generators output audio.
AUv3 instruments can generate sound and be controlled by MIDI routed to it by the AUv3 host app.

AUv3 effect apps apply effects to audio routed to it and output the effected sound.
Some AUv3 effects apps can be controlled via MIDI routed to it and output the effected sound.

AUv3 MIDI apps output MIDI which can be routed by AUv3 host app to other hosted apps, MIDI controlled hardware (e.g. a synth like a Korg Minilogue, or non-hosted apps.
AUv3 MIDI effect apps can receive and modify MIDI which can then be route by the AUv3 host like AUv3 MIDI apps.
MIDI which can be routed by AUv3 host app to other hosted apps, MIDI controlled hardware (e.g. a synth like a Korg Minilogue, or non-hosted apps.

AUv3 apps can use several stereo input and output busses (for multi-output or side-chaining) - this feature also needs to be supported by the AU host
List of AU multi-bus audio units and hosts

There are currently no known bugs similar to IAA zombies for AUv3 apps as all instances of an AUv3 app are closed when you closed the AUv3 host app they're hosted in.

AUv3 developers can choose to expose their parameter controls which can be controlled via MIDI by AUv3 host apps which host them provide this functionality is supported by the AUv3 host app.
AUv3 parameter automation. Some AUv3 hosts support the ability to record the movements of the GUI controls (e.g. knobs and sliders) of a hosted AUv3 app. This provides a much finer level of control than MIDI which may be significant depending upon the particular apps, setup, and musical goal you're trying to achieve.