If you have an ECE (European) or USA car, and you want radio always fullscreen, select "radio always fullscreen". If you want to see if split screen works for your radio, select "Radio splitscreen enabled".

If you have a Japanese car with Japanese radio, select C23 Japan

If you have an Oceania car select C23 Oceania

But it's easiest just to try one of the first two and see what happens. Much quicker and simpler than death-by-1000-questions