Diesel fuel gels because the paraffin wax in it solidifies in cold temperatures, causing the fuel to thicken and clog fuel lines and filters. This happens when the fuel's temperature drops below its cloud point, where wax crystals begin to form, and then its pour point, where the fuel can no longer flow. Gelling will lead to complete engine stoppage. (usually in the most severe weather conditions, and the worst possible place)  DFL protects to minus 50 degrees.