The term of decision point is confused here with decision altitude (DA).
i.e. the mentioned decision point refers to the approach ban, meaning that the approach can be continued past this point regardless of what happens to the RVR.
Say, you do the CAT III, and before this point (can be the Glideslope Intercept point / Final Approach Point - can be at 5NM final) the RVRs are the minimum 200 meters; if after this point the RVRs decrease too, say, 100 meters, it is at your discretion to continue the approach to your Decision Height of 50' (to "Look and see"). If Wx then doesn't improve and you make a GA, you'llnot be able to even start the approach next tim, since the RVR are below the minima needed at your "gate".
Edit: Changed Desision Altitude into Decision Height, for the CatIII example. Thanks JAR!