CAT II/III landing

Hello everybody,

Having recently visited an International Airport, I have got one more question ;-)
Probably you can help me clarify something regarding the CAT II/III marking strips. When there are unfavourable weather conditions like fog, leading to a Category III landing approach, I guess those strips are rendered nearly invisible!?
Is the respective CAT - marking strip possibly illuminated?
Are those markings just an indicator showing that nobody is allowed to cross the respective strip on the taxiway while the landing manoeuvre is being carried out?

To put it in a nutshell: What exactly do those strips indicate and how do you know which one is currently active?

Thanks in advance for your help,
John