One of the elements of the AutoCAD user interface you have constantly in front of your eyes is the icon of the user coordinate system (UCS). But only few users really know all shapes and options of this icon. It has also changed in the AutoCAD history.
The way the UCS icon is displayed is controlled by the UCSICON command and depends on the current settings of the coordinate system, view direction, visual style and other parameters.
Examples of the UCS icon and a legend to the individual cases:
-  2D, world coordinate system (WCS), display set to "origin" and the icon is displayed at origin
-  2D, world coordinate system (WCS), display set to "origin" but the origin is off-screen
-  2D, world coordinate system (WCS), display is not set to "origin"
-  2D, a user coordinate system (UCS) is active, display set to "origin"
-  2D, a user coordinate system (UCS) is active, display is not set to "origin"
-  3D, general world coordinate system, display set to "origin" and the icon is displayed at origin
-  3D, isometric view on the world UCS
-  3D, a general user coordinate system, the Z-axis points downwards (dashed)
-  3D, a special case of a user coordinate system, view to the XZ-plane
-  the old "2D" style of the UCS icon from version 2000 and older (can be set in UCSICON), here the plan view of the world coordinate system ("W" was localized in older localized versions of AutoCAD)
-  you can customize the look of your UCS icon through the UCSICON command (cones, width, color - see below)
-  in 3D visual styles, the UCS icon is displayed shaded and in color; you can distinguish axes even with suppressed axis labels - XYZ = RGB
Quick hint: if you can see the box, you are in the world UCS, otherwise you are in a user UCS
The dialog of the UCS icon settings invoked by the UCSICON command with the option Settings (Czech version shown):