Residential , industrial, transportation energy source as a residual refinery product. Various types are classified by their specific gravity . The higher the specific gravity, the higher the category number. Heat energy in residences comes from burning Number 1, 2, and 3. Industrial use involves Number 4, 5, or 6. Ship fuel is Number 5 and 6, called bunker oils. These fuels have a flash point above 40
Black’s Law Online Dictionary
