Here is my personal testimony regarding beds that cannot be combined:
Our first cruise was in 1992 on the old Westerdam of Holland America. It was a big group cruise of several hundred people and was arranged by some large agency in Atlanta that was inpersonal and just plugged people into cabins without asking or providing specifics. We didn't know anything about cruising at the time. When we got to our cabin we discovered that it was one where the beds were attached to the walls and could not be combined. Not a lot of fun for a couple's first cruise. That agency didn't really care about us as individuals, but they had the exclusive rights to that group cruise.
I told the story to a cruise specialist I met a couple years later. She almost immediately asked if we were in an F category cabin on upper promenade deck. She knew.