Commonly known as bloom effect, Scattering occurs when the light is scattered inside the lens before reaching the film.
Note that the Scattering effect (as Diffraction as well) will be more visible the stronger the light source is. For example, if the camera sees the sun or a strong emitter, these will have a stronger scattering/diffraction effect. Indeed, the Scattering and Diffraction effects (bloom and glare) will still be visible in the scene even if no direct light sources are seen by the camera, but the effect will be weaker.
Do not enable any Simulens setting before starting the render as these settings need to be calculated for each image update in the main render window. This will add extra render time and also increase RAM usage during rendering, especially for high resolution renders. Instead, wait until the render has reached an acceptable SL for viewing the Simulens effect, stop the render, apply a Simulens effect to give you an idea of its influence, then disable the Simulens again and resume the render. For adding Simulens effects to a range of MXI files after the render is completed, use File>MXI Batch Processing. This tip does not apply to the Devignetting effect which does not use more RAM or calculation time, you can enable it before starting the render.