When you select a layer (i.e. Contours, Post, Base, etc), the Coordinate System tab shows the coordinate system for the data or grid file used to create that map layer. This is the coordinate system of the source data or grid file, not necessarily what you want the map to be displayed in. If the source data file had a reference file with this information, then it is automatically filled in. If the source data file did not have a reference file, then the coordinate system Name for the layer shows Unreferenced local system. Click the Set button to set the original coordinate system for the source data for that layer. Once the coordinate system is set for the layer, it should not be changed unless you change the source data or grid file.
When you select a Map in the Contents window, the Coordinate System tab shows the coordinate system that the map will actually be displayed as in the plot window. Click the Change button to change the coordinate system for the map frame to any coordinate system you want. When you change the coordinate system for the map frame, all map layers will be converted from their source coordinate system to this target coordinate system for display.
Updated January 16, 2017