The UCS is a moveable Cartesian coordinate system that establishes the XY work plane, horizontal and vertical directions, axes of rotation, and other useful geometric references1. You can change the UCS origin and orientation for convenience as you specify points, enter coordinates, and work with drawing aids1.
The face option sets a new UCS based on a selected face of a 3D solid object2. This option will align the XY plane of the UCS with the selected face and show its correct size and shape2.