A319 First Officer reported visibility going below minimums outside SIBLY during a CAT IIIB ILS to Runway 27R at ORD. Captain believed this was acceptable and continued to autoland.

Narrative

On this flight the weather at ORD went well below the forecast and we entered holding on the PAITN Arrival. Weather was reported 1/8 SM fog; 500/600/600 RVR Runway 27R. ATC asked us what RVR we needed to land on 27R. We told them 600 RVR was our limit. During the hold the Captain briefed a CAT III autoland to 27R ORD. The RVR rose to 600/600/600 for 27R and we were released from the hold and vectored for the approach. Approach Control cleared us for the ILS 27R and we intercepted the glideslope at 4000 FT; over BONZO. Between BONZO and SILBY (OM) approach reported the RVR for 27R 500/600/600. I told the Captain that I didn't think we were legal for the approach. He disagreed and said that since we were already established on the final approach segment; that we could continue the approach. We switched over to ORD Tower and they reported 500/600/600 RVR. We continued to a successful autoland and taxied to the gate. After the flight we discussed what happened. I was under the impression that the final approach segment begins at the glideslope crossing altitude. The Captain said that it begins were the fan is depicted on the plan view of the localizer; in this case; outside of BONZO. This made us legal for the approach. The Captain contacted the Flight Manager after the flight and he concurred that he was correct. There was a lot happening in a short period time in the cockpit; so we were both pretty task saturated. This added to the confusion of the situation. In the future I will add this scenario to an autoland brief to avoid confusion at a critical phase of flight.