The standard approach for CFS in Radiance is to precompute the light passing through each window or skylight, treating it as a "white box" in the final rendering, otherwise known as a "secondary light source." The original mkillum program was written to convert a CFS, described as geometry and materials, into a secondary light source with a luminous intensity distribution corresponding to a particular exterior condition (that is, sun, sky, obstructions, reflections, etc.).
This is why the default method computes a secondary light source (12), leaving the final rendering phase to create a realistic view out the window.
Since we know that windows are important contributors to illumination, we can turn them into secondary light sources, precomputing and smoothing their output distributions for efficient rendering.