This message is a mechanism for the physical device driver to return status, indicate that the buffer has been consumed, or indicate that a new buffer is needed.
When status is returned, you will know whether the playback or record was successful or if there was an error condition. Error conditions for playback are known as underruns, which means the device is not getting data fast enough. Error conditions for records are called overruns, which means the device is generating data faster than empty buffers become available. This might result in a data loss situation. For example, the device is constantly bringing in data from the analog ports of the adapter and converting it to digital data. If the device does not have any empty buffers to store the digital data, the data is lost.
If you report an underrun under Warp and want the stream handler to pause the device, you should report the ERROR_DEVICE_UNDERRUN as well as the SHD_WRITE_COMPLETE flags.
The VSD must return buffers in the order they were sent and should not hold on to any buffers. Failure to return buffers could result in application hangs.