They are designed for different applications. Heavy-duty chain block is designed for industrial-grade professional overhead lifting. While light-duty chain block is designed for lashing purposes, not recommended for lifting. The light-duty chain block can be used for life unless you know exactly the weight of the loads and any overweight lift is absolutely prohibited. Heavy-duty chain block has a minimum of 4 times safety factor and has been typing tested with life cycle test. It's reliable and durable for high-frequency heavy loads.