A face coordinate system is tightly coupled to a particular face of a solid. Its orientation and position automatically adjust when the associated face changes, perhaps as a result of the corresponding solid being edited or transformed. Face coordinate systems are very useful when defining one solid to be located/oriented relative to another solid.
To create a face coordinate system, on the Structure ribbon in the Working Coordinate System/Plane group, click the button and choose New Face Coordinate System.
See the command hint for exact details, but this action prompts you to choose a face followed by the origin and orientation of the face coordinate system using snap points on the face. Analyst automatically determines which component is normal to the chosen face to best align with the world coordinate system, but this can change after creation in the face coordinate system properties.