Per the 1994 Agreed Framework, North Korea received 500,000 tons of heavy fuel oil annually, and the recent deal calls for one million tons to be delivered, although the first shipment will be only 50,000 tons, which South Korea is funding (the rest likely will never follow, but that’s another story).

So what exactly is heavy fuel oil? The short answer is, “what remains of the crude oil after gasoline and the distillate fuel oils are extracted through distillation.” Distillate fuel oils refers to diesel, kerosene, and the like. There are six classes of distilled oil, fuel oil being the lowest two grades, five and six.

One might think that heavy fuel oil is therefore not very valuable, and while it is worth less than the higher grades, its primary use is for being, “burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power.” Perfectly suited to the antiquated Soviet power, production, and heating plants in North Korea? Actually, no:

. . .because their power plants were designed to burn low-sulfur Korean coal rather than high-sulfur heavy fuel oil. The sulfur content leads to the formation of corrosive acids, which further cripple decrepit, Soviet-designed plants—damaging parts that are no longer available.

But fuel oil can’t be used for any overtly military purposes.

More from Wikipedia:

Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash point of approximately 40 °C (104 °F) and oils burned in cotton or wool-wick burners.

[…]

Fuel oil in the United States is classified into six classes, according to its boiling temperature, composition and purpose.

[…]

Price usually decreases as the fuel number increases. No. 1 fuel oil, No. 2 fuel oil and No. 3 fuel oil are referred to as distillate fuel oils, diesel fuel oils, light fuel oils, gasoil or just distillate. For example, No. 2 fuel oil, No. 2 distillate and No. 2 diesel fuel oil are almost the same thing.

[…]

No. 3 is a distillate fuel oil and is rarely used. No. 4 fuel oil is usually a blend of distillate and residual fuel oils, such as No. 2 and 6, however, sometimes it is just a heavy distillate. No. 4 may be classified as diesel, distillate or residual fuel oil. No. 5 fuel oil and No. 6 fuel oil are called residual fuel oils (RFO) or heavy fuel oils. As far more No. 6 than No. 5 is produced, the terms heavy fuel oil and residual fuel oil are sometimes used as synonyms for No. 6. They are what remains of the crude oil after gasoline and the distillate fuel oils are extracted through distillation.