To Align the UCS With an Existing 3D Object
Click the UCS icon, and then click Move and Align.
Drag the UCS icon over the portion of the object with which you want to align it.
Click to place the new UCS.
Related Concepts
About the User Coordinate System (UCS)
About Controlling the User Coordinate System (UCS)
Related Reference
Commands for the User Coordinate System (UCS)