Diesel fuel is not the same as heating oil. Diesel fuel has a different formulation than heating oil. Additionally, fuel oil is dyed red and not allowed for highway use, while diesel fuel has a road tax applied at the point of sale.
There are a number of classifications for heating oil that distinguish various formulations and attributes. Heating oil is petroleum-based and is derived through a refining process. Diesel fuel can also be distilled from petroleum; however, it can also be created from biomass or synthetically. The American Society for Testing and Materials documents the standards for heating oils in ASTM standard D396.