fd60a641_Co_ordinate_Systems___CADTutor__Method_of_UCS_determination
[Object] Line [Method of UCS determination] 
The endpoint nearest the pick point becomes the new UCS origin. The new X axis is chosen so that the line lies in the XY plane of the new UCS.
AutoCAD Tutorial | User Co-ordinate Systems | CADTutor
Method of UCS determination