CO2 is an upgrade option with extra sampling hose included.
When you purchased the high concentration CO2 option, you get an extra hose. This option allows 0-100% CO2 values, on top of ppm that the base unit does.
If you test 100% CO2 first and then try to measure ppm values of CO2, it will take a while to purge the CO2 from the hose since CO2 is a sticky gas. An extra hose allows the user to have a dedicated hose for high concentration CO2 testing.