The Design Standards (for example,
FAR 25.1401) require a minimum volume coverage of the lights .. the OEM picks (generally at least two) spots which. in concert, achieve the design requirements.
At the end of the exercise you will have picked up a new word;
steradian - a solid angle at the center of a sphere subtending a section on the surface equal in area to the square of the radius of the sphere
isn't PPRuNe a wonderful sandpit of bits of information you really didn't set out to find ... ?