> What is Unit0 exactly

AHI has two different APIs, a high-level DOS device one with Units 0...3 and a low-level library one with the Music Unit. The main difference is that the Music Unit offers the best audio quality but works in an exlusive way by allowing allocation by only one program at a time. Programs using device units can play concurrently but at the cost of quality loss.

> What is the difference between Unit0 and AppleI2s... devices.

The Music Unit can be either set to the audio hardware driver (exclusive output, high quality) or redirected via the device.audio driver to one of the device units (concurrent output, lower quality). The redirection enables any program that uses the low-level AHI API to act like a program that uses the high-level AHI API, with the mentioned pros and cons.

> What is the difference between ++, Hifi and normal devices.

http://www.lysator.liu.se/ahi/v4-site/files/ahi/doc/ahiusr_18.html