SU-1 is a digital to digital conversion.
What does it convert ? Well, it take in USB signals, and convert it into I2S signals whatever the USB signals may contain (PCM, DSD...etc..)
What is the I2S signals ? I thought DAC only take USB in ? Well, I2S signals is a processed and separate timed clock signals, it is called Inter-IC Sound (IIS). Your IC chips AKA DAC-chip is communicate by this Signals. Then what about the USB in on the DAC ? Exactly this, in digital world, you need digital interface (USB, Coax, AES, IIS...etc..). USB is just one of the input on the DAC, but whatever input it takes in, it would need to have an according interfaces to interprete this incoming digital signal into IIS signals. So, in short, your USB input in the DAC is simply USB-Digital to Digital Conversion. It will become IIS before it get to your DAC.
So why do we use DDC (digital to digital conversion) ? Well, because more than often, the USB within your DAC is poorly implemented with only acceptable clock oscilliators and or power supply. The SU-1 is a dedicated DDC. It has separated ground isolation, linear power supply with ultra low noises, and other features to improve the sound quality for your digital files