I think a lot of people confuse what is a DDC
1) DDC : USB in -----> non USB out, e.g. I2S, coex, opt, AES. No OS is loaded and can't function as network end point. (USB out don't function here)
2) Streamer : Network in (Rj45) -----> USB, I2S, coex, opt, AES output, need OS to load up (running network end point) and running to process data from network, common end point protocol like UPNP, DLNA, Roon, HPQ NAA, Squeecelite etc.
Red can function as DDC only or Streamer only at any one time. As above already discuss way to switch function by whether there's USB out port connected.
During power on:
USB input port not connected with SD card loaded with OS ---> Streamer mode
USB input port connected ---> DDC mode