Gas Oil is a middle distillate liquid refined from crude oil. It has many different names such as diesel and a variety of uses. Gas Oil is a “dual purpose” fuel. It has a high calorific value (heat content) and is thus recommended as a boiler fuel in industrial or agricultural installations with pressure jet burners e.g. grain dryers. It can also be used in a diesel engine fueling off-the-highway equipment such as farm tractors, construction equipment, railway and marine engines, and stationary diesel engines or generators. It can be called: Gas Oil, Red Diesel, Farm Diesel, ULSG, Class A2 or Class D, 35 second burning oil, Marked EN590.    
A lot of the confusion around Gas Oil, and the variety of different names, has come both from its variety of different uses, and changes to product specification over the last few decades.