Enable Visability Graduation and Smoothing option

Read the full manual today and after making some better selections I am very pleased with the weather depictions. I guess the features I really like are reduce visability in cloud which for the first time in FSX is preventing me from seeing the ground when above a cloud layer. Also, I think the fog generation is fantastic.

I do have a question about the feature "Enable Visability Graduation and Smoothing" and the associated altitude in ft. If I set this at say 18,000 and the metar cloud layer is overcast at 4000ft, does this mean you are in the clouds until 18000ft with a gradual dissipation of the overcast layer? If so what happens if you do not check that option and what is the best value to put in if you do use it?