Under the ULSD regulations, a minimum of 80 percent of the diesel fuel produced for highway vehicles must be ULSD with a maximum sulfur content of 15 parts per million (ppm), while the remaining 20 percent may be low sulfur diesel fuel (LSD) with a maximum sulfur content of 500 ppm. However, beginning June 1, 2010, all highway diesel fuel must be ULSD. Pumps used to dispense diesel fuel into motor vehicles must be labeled as to the type of diesel fuel being dispensed. The 80 percent ULSD production requirement is intended to ensure that ULSD is available for use in model year 2007 and newer diesel vehicles, which require use of ULSD.
http://epa.gov/oecaerth/civil/caa/ultralow-sulfurdieselfuel.html
Locomotive, marine and non-road diesel fuel standards begin at later dates (except in California).
http://www.clean-diesel.org/nonroad.html
I wonder why refineries don’t just produce one product.
Less likely to have a mistake or contamination, fewer storage issues, costs are the same...