Red provides the heading to the left of which the light disappears. Green provides the heading to the right of which the light disappears. Blue provides the pitch below which the light disappears. Alpha provides the pitch above which the light disappears.
RGB color is set to white. Alpha is set to brightness or 0 for off.
The fresnel light provies a rapid-cutoff when the viewing angles are exceeded. Unlike normal directional lights with rescaling, the fresnel light is full brightness, until you reach the cutoff angle, at which point it almost immediately disappears. Like a real-life fresnel the sharp cutoff can be used to create guidance lights.
The only difference between the horizontal and fresnel fresnel datarefs is the cutoff speeds are tuned differently.