To support more consistent element positioning and intersection logic, Wall Reference Line options have been renamed and expanded. Instead of Left, Right and Center, they are known as Outside Face, Center and Inside Face. (New core-based reference line options are also available; these include Core Outside/Center/Inside).
The “flip” status means that a selected Wall is flipped (or mirrored) along its reference line. The reference line stays in place, but the body of the Wall flips to the other side.