Great USB audio summary.
"Most modern DACs with USB input implement "Isochronous transfer mode with asynchronous feedback". The isochronous transfer mode of USB is the one that guarantees transmission bandwidth but does not support data retransmission (in response to data packet CRC errors). In contrast, the bulk transfer mode of USB (the one used for USB flash drives & other storage devices) has a packet retransmission capability but no guaranteed throughtput. Isochronous transfer mode favors data transmission regularity over integrity, whereas bulk mode is the opposite.
In USB, strictly speaking there is no asynchronous mode, but rather "asynchronous feedback", so for the DACs, it is not a choice between implementing isochronous or asynchronous.
Asynchronous simply means the receiver (being the USB DAC) will send information back to the source (PC, streamer, etc.) to slightly speed up or slow down the data transmission, to prevent buffer underruns or overruns at the DAC side. Essentially the USB DAC controls the pacing of the data.
With good USB signal integrity, achieved with good quality and not-too-long USB cable, data corruption should be a non-issue. The isochronous transfer mode with asynchronous feedback is the correct way to send audio over USB, and tremendously helps prevent audio dropouts due to interruptions in the data stream."
Scan80269 as quoted from audiostyle.com, April 2017