ADAT Optical is 8 channels of audio ranging from 16-bit 44.1KHz to 24-bit 48Khz in quality.
ADAT SMUX II allows 4 channels of audio at 24-bit 96KHz over the same optical cable. By sacrificing 4 channels of audio, the optical cable interface is able to handle a higher signal quality audio capture signal.
ADAT SMUX IV allows 2 channels of audio at 24-Bit 192KHz over the same optical cable. By sacrificing 6 channels of audio, the optical cable interface is able to handle a higher signal quality audio capture signal.