This ReadMe file for the Apple USB Audio Drivers on OS X contains
a brief overview of the files this directory:

AppleUSBAudioEngine contains the classes AppleUSBAudioEngine which inherits from IOAudioEngine.
This class is responsible for streaming the audio.  It sets up the framelists and requests
DMA IO from the IOUSBInterface and IOUSBFindEndpointRequest, using IOUSBLowLatencyIsocFrame
and IOUSBDevRequest, then receives completion from IOUSBCompletion.