This is right out of their software.They explain it well.
In most applications, Speed/Density mode will provide the best overall performance for the least amount of tuning effort from inexperienced tuners. Load Indexed Speed/Density mode is something unique to your FAST XFI. In this mode, a “load” variable is formed from the ratio of manifold pressure to barometric pressure. This “load” variable is used to index into the VE, Target Air Fuel, and Base Spark tables. Barometric pressure is sampled from the MAP sensor prior to start up. Its advantages are for users that may drive in areas of vastly differing altitudes. For example, full engine load at sea level is approximately 101 kPa, but full engine load at 3000 feet above sea level is significantly less in kPa. This means that in regular Speed/Density mode the point of full engine load in the VE table is in a different spot for each of the two altitudes. Load Indexed Speed/Density takes that difference out of the equation and makes the point of full load the same no matter what the altitude, ultimately making the tuning process easier. Neither mode will produce more power than the other
