Core-Based Wall Reference Line

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.
When you migrate to ARCHICAD 20, Wall Reference lines will be redefined as follows:
AC16 Left = AC20 Flip on + Outside Face
AC16 Left + Positive Offset = AC20 Flip on + Outside Face + Positive Offset
AC16 Center = AC20 Flip off + Center
AC16 Center mirrored = AC20 Flip on + Center
AC16 Right = AC20 Flip off + Outside Face
AC16 Right + Positive Offset = AC20 Flip off + Outside Face + Positive Offset