D2 is a refinery abbreviation for Gasoil. It is the second distillate from the crude oil, and can be used without reformers and additives. So, the first engines used D2 as fuel - before petrol cars as we know them today was invented. That is because the engine invented by a German called Diesel, requires no spark plugs. The diesel engine will ignite and combust when the pressure increases so that the heated "plug" makes it explode. Here we get the name "Diesel" - since the same principles are used in diesel engines today. However, automotive diesel that you fill has additives that the refinery will add to make the engine more efficient and also easier to start in the winter. Diesel changes "flash point" in the winter. It also has additives to absorb water that condense. If you use summer diesel in the winter, you will get better mileage, but your fuel pipes may freeze and can also burst, and the wax makes the diesel flow thicker.
ISO has a standard for D2 that most of oil companies use as their reference.
In the U.S. it is ANSI that has defined the US national standard for D2, according to proposals from the ASTM, API and EPA.
In Europe and elsewhere there are similar national variants, e.g. in Germany set by DIN, and in Russia and CIS by GOST.