The arrow shows the flowing direction of gas. The diameter is 2.5 MM, and the BTU should be around 78,000 (NFPA-11). There are six small holes around the yellow center hole. The yellow center hole is for L/P Gas Conversion.The orifice is converted L/P when the holes are covered with a special washer (included). It is set to natural gas when the washer is taken out and the six holes are open. MUST NOT BE OPERATED CLOSER THAN 6" FROM OPEN AN FLAME.