Mazut is a heavy, low quality fuel oil, used in generating plants and similar applications. In the Western Europe, mazut is blended or broken down with the end product being diesel.
Mazut-100 is a fuel oil that is manufactured to GOST specifications, for example GOST 10585-75 or 99. (GOST is the Russian system of standards). Only Russian Mazut M100: The Mazut M100 is supplied according to the Russian technical specification GOST, section for fuel oil, mazut sort 100.

This kind of oil is graded as the heavy furnace oil. The product is produced from the remains of raw oil processing. This kind of mazut is produced only from the low sulphur raw oil. Origin country for this kind of mazut is only Russia. Mazut is almost exclusively manufactured in the Russian Federation, Kazakhstan, Azerbaijan, and Turkmenistan. The most important thing when grading this fuel is the sulphur content.

It is very limited volume of this mazut available for export from Russia because of the following reasons:

There is limited number of producers in Russia, who produce this kind of mazut. All of them are the big Russian oil companies, like Lukoil, Rossibneft, etc.
Minimum half of the produced volume is sold on the domestic market of Russia and CIS.
Most of the volumes for export are sold according to the state quotes to the state companies abroad.
The remaining export volumes are sold by the associate commercial organizations of the oil producers to the highest bidder.

This product is typically used for larger boilers in producing steam since the BTU content is high.

The most important consideration (not the only consideration) when grading this fuel is the sulfur content, which can mostly be affected by the source feedstock. In the United States and Western Europe, mazut is blended or broken down, with the end product being diesel.

Mazut 100-75 VLS and Mazut 100-99 Grade I are actually the same thing. GOST merged the old classifications of 75 and 99 into a new seven grade classification, all under 100-99.

For whatever reason, many people still use the old 75 classification; particularly the Chinese.

The grades are represented by these sulphuric levels:
Very Low Sulphur is mazut with a sulphur content of 0.5%
Low Sulphur is a mazut with a sulphur content of 0.5-1.0%
Normal Sulphur is a mazut with a sulphur content of 1.0-2.0%
High Sulphur is a mazut with a sulphur content of 2.0-3.5%.