fd60a641_Co_ordinate_Systems___CADTutor__Method_of_UCS_determination
[Object] 3D Face [Method of UCS determination] 
The new UCS origin is taken from the first point, the X axis from the first two points, and the Y positive side from the first and fourth points.
AutoCAD Tutorial | User Co-ordinate Systems | CADTutor
Method of UCS determination