the red ring is to calibrate. So in theory you set it to the desired temp which let’s say is 75. If in 24 hours the tank is at 77. You turn the red dial to 77 and then re-adust the Temp(blue knob) it’s should be very accurate after that.
Edit: I finally figured this out after people always mentioning how they like the Eheims for the ease of calibration. It took me a week or two to understand that the red knob does something. Always pays to read instructions lol