When using Solar lighting, the light rays are present throughout the model, not just in the vicinity of the design geometry. For efficiency, you can create one or more sky openings that restrict the calculations to the light that passes through the sky openings only. You should use sky openings in particular where you have an interior space illuminated from "outside" by Solar lighting. For exterior scenes also, you can use sky openings to focus the processing on the region of the model where the design geometry is located.
When one or more sky openings are present in a model, all particles from Solar light are restricted to pass through the sky openings and not be dispersed throughout the model. This speeds up the rendering process by letting you use less particles than would be required where no sky openings present. Both images below are Particle Traced, with the same number of particles. On the left, the image does not have a Sky Opening, so many particles are "lost" to illuminate outside the building. When a Sky Opening is created (the same size as the window), all particles are shot through the Sky Opening and, in this case, through the window, producing a better image.
With Sky Opening active, all particles are shot through the window.
Sky/Solar Lighting shining through a window, without Sky Opening(left) and with Sky Opening (right).
In real-life, light coming through a window comes not just from the Sun, but from the entire surroundings. You can simulate this by turning on Sky Dome global light. During processing, light is assumed to be emanating from throughout a hemisphere. When a sky opening is present, the calculations are restricted to only that part of the hemisphere seen through the sky opening.
The image below has been Particle Traced using the same number of particles as the two previous images (above). As you can see, it is much brighter because the light is not concentrated from only the direction of the Solar Light.
With Sky Dome added, the image is brighter.