If you’re seeing that, it’s because the four output channels of the SHD are made available to the computer over USB. In your computer’s software, these are therefore seen as inputs.
This feature of the SHD series/ DDRC-24/2x4HD series is a handy feature to allow use of the platform as a USB ADC platform so you can “record” analog/digital content to your PC. E.g. transferring old content to digital versions.