The fuel line connects to the fitting on the main body on the carb. The remaining fitting on the regulator cover plate may be used or not, depending on your installation. In some cases, excessive cowl pressure caused by poor cooling airflow will make the engine go rich at certain airplane attitudes and airspeeds. Connecting that fitting through a line routed back in the fuselage and connected to something like a balsa box or pill bottle can eliminate the effects of that cowl pressure.