Marsha,
I took that that RS tour previously and the tour went to the two camps: The main camp (Auschwitz I) and Birkenau (Auschwitz II). We had a similar disclaimer (when available) which I believe is just a disclaimer that sometimes Birkenau isn't necessarily open. From what I understand it is sometimes closed when there are official ceremonies or head of state visits. The Auschwitz-Birkenau website supposedly lists in advance the closures. (https://www.auschwitz.org/en/). I think the closures are relatively infrequent.
Assuming they haven't changed the itinerary much, Your tour will take you first to Auschwitz I, which is a pretty comprehensive and somewhat emotional tour through the living spaces with lots of stories from the guides. Birkenau, is more subdued and is a self guided walk. Much of the camp structures are gone, however there are a few and some representative structures. Lots of signs to read. It was interesting and definitely worth it.. Much less to see there than the main camp... But both are definitely musts..
As a side note, I believe RS has added a day to the tour. When we took it, we visited Auschwitz after leaving Krakow on the same day we drove to Eger which was a super long day (didn't get in until 9 PM since we stayed for several hours at Auschwitz), but now they just stay another night in Krakow before driving to Egar which was a drastic improvement. I know many people noted that in the tour reviews and am glad that RS addressed it....
I hope you have a wonderful tour, it was one of our favorites (we've been on 7 now). Although they used to call it the Best of Eastern Europe, but I think they must have changed the name to "central" for marketing reasons after the Ukraine/Russia conflict started.