The general idea as I understand it is that the structural elements of the frame perimeter cannot move relative to each other, and the bumpers have to be locked in place on the parts of the perimeter they cover.
The one time the non-articulated thing has come up for me as an inspector, the team had built their frame with a joint in it so that they could flex the frame from a flat rectangle to more of an A frame when going over an obstacle on the field. That was not ok, and they had to lock that joint in place to compete. (This was one of those “I’m just going to go ahead and get the LRI right now, because I know you’re going to want to appeal” kind of moments. I felt bad about it because they’d clearly spent a lot of time making that happen, but it was definitely in violation of the requirement.)