Input of a refinery is crude oil, output is a number of different products. These products, like fuel oil or gasoline, are produced – in simple terms – by heating and evaporation of crude oil. But: As a result of refinement there is a residue. You can refine this residue, too, but this is not our topic.
The residue, you won’t believe it, is called residual oil. Or heavy fuel. However, it is full of metal, sulphur and nitrogen compounds. Residual oil is literally the least. But still it is a great demanded product.
As you can imagine that’s the case because of its absolutely attractive price, a good explanation for the high amount of inland and seagoing vessels running on heavy fuel.
*) But marine fuel is not always bunker oil.