fd60a641_Co_ordinate_Systems___CADTutor__Method_of_UCS_determination
[Object] 2D Polyline [Method of UCS determination] 
The Polyline's or Polyline arc's start point is the new UCS origin, with the X axis extending from the start point to the next vertex.
AutoCAD Tutorial | User Co-ordinate Systems | CADTutor
Method of UCS determination