Pet reg tractors are vehicles that meet international standards for the safe transport of hazardous materials including petroleum.
In very simple terms, pet reg tractors are vehicles that are approved for hauling hazardous goods, with petroleum being the most common. The regulation in question stems from the ADR treaty, aka the European Agreement concerning the International Carriage of Dangerous Goods by Road, which the United Nations Economic Commission Federation for Europe established back in 1957.
The main article of this treaty, and the one that most directly relates to pet reg tractors, talks about how hazardous goods must be packaged and sets up regulations for the construction and general usage of vehicles carrying hazardous materials. In addition to the goods and vehicles themselves, a driver operating a vehicle carrying potentially dangerous materials also needs to have an ADR license.