This is the ability to aggregate multiple audio interfaces or hardware connections into a single virtual device. Some time ago, OS X gained a rather interesting ability which addressed a fairly common problem for musicians.
Developers are able to use these standards when making hardware and software, and the result is an infinitely more reliable way to transfer data.
This all changed with OS X, which uses CoreAudio and CoreMIDI frameworks built in at system level. These were bolt-ons, and together with OS9's inherent stability problems related to Extensions (I still shudder when I think of how often Macs used to crash compared to today), meant you were more or less at the mercy of whoever had written the drivers for your device.
Anyone who has been making music with Macs for more than a decade will remember the 'bad old days'� of OS9, when audio and MIDI streaming in and out of a computer meant relying on specially written drivers and the OMS MIDI system.
