The Map | Coordinate Conversion command is used to change the projection assigned to the input data or to “calibrate” an unprojected map (either to calibrate/georeference an imported vector file, or to calibrate/recalibrate a project that has both raster and vector data in it).
1. If you import an AutoCAD DXF file and you know the coordinates in the DXF file are in UTM, then you can import the file as UTM, and then use Map | Change Projection to change the display to some other projection (like State Plane).
2. If you import an image georeferenced with a projection (or download a WMS image), and want to change the projection from UTM to State Plane, use the Map | Change Projection command.
3. If you import an AutoCAD DXF file and the coordinates in the DXF file are some random special coordinate units unique only to that particular site (say they are meters based of a 0 baseline). You would use the Map | Coordinate Conversion command to georeference the DXF file to UTM, then use Map | Change Projection to change the projection to State Plane.
4. If you import a georeferenced image, digitize some points/polylines/polygons on it, then realize the georeferencing is 50m off, you would use Map | Coordinate Conversion to recalibrate the project.
Updated May 31, 2018