From the Toolbar: Find the Align UCS to Selected Entities Tool icon on the UCS Draw Plane Toolbar (color coded purple) and Click on the tool. The tool is now active and ready for use.

 
From the Shortcut Key: This tool does not have a default shortcut key.

 
From the Command Line: Type "snap_ucs_to_entities" into the Command Line, at the top of the notification window, and press the Enter/Return key.

This tool is a Stacked Tool. If there is no selection prior to the activation of this tool the Selection Tool will become active and allow the selection of the entities that will be used to define plane.

1.        Select entities to define plane.

2.        Press Enter to finish the Selection Tool. The tool will try to find the plane defined by selected entities. If all selected entities are on the same plane then the tool aligns the active UCS to this plane.

Reset:

After changing the active UCS the tool will automatically reset. You can either continue on with other tools or use the align UCS to selected entities tool again.

Escape (Esc): Cancels current tool and activates the default tool (Selection Tool).

Space: Resets this tool.