Residual Fuel Oil, often referred to as "bunker fuel," is a heavy, viscous oil that remains after the distillation of crude oil. It is primarily used in large ships and industrial boilers due to its high energy content and low cost. This type of fuel is not as refined as other petroleum products, making it less suitable for smaller engines. Due to its high sulfur content, the use of residual fuel oil has raised environmental concerns. Regulations, such as those from the International Maritime Organization, aim to reduce emissions from ships using this fuel, promoting cleaner alternatives to protect air quality and marine life.