Yes, some of us here know that
, but that's just a number on the flight plan, telling ATC the lowest RVR you can ever accept, anywhere in the World.
Remember (?) you are also restricted by the minima associated with the runway and the approach aid..so if it was down to 300 metres in fog in Sydney and the published minimum allowable visibility/RVR for that runway was 550 metres or even 34 Left's 1.5 km (thanks Deano - see below) then putting it very simply you're not allowed to land, if fact you can't even make an approach to below 1000 feet; the 100m Flight Plan figure is pretty much irrelevant.