I guess this regulation specifically addresses the case of shallow fog - where the in-flight visibility is unlimited, but the soup starts below 100'. Still, the basic requirement for commencing a visual app is to be able to see the runway throughout the maneouvre. The RVR800 rule is a restriction to ensure adeqate visual reference during landing.
Last edited by Stuck_in_an_ATR; 14th February 2013 at 08:05.