2 Points
This command creates a new UCS and sets the orientation of the X-axis from 2 user clicks.
No selection is needed to launch the command.
Launch the command.
Click the first point according Define points. It will correspond to the UCS's origin.
In the same way, click the second point to define the orientation of the X-axis.
This will not change the orientation or direction of the Z-axis. The Z-axis will stay in the up direction.