Projection files (.prj) are used to store the projected coordinate system defining the parameters and the linear units used in the projection with the description of the geographic coordinate system. In ArcGIS Desktop 10.3 and previous, it is possible to create a custom .prj file with additional comments and special characters such as asterisk (*), as shown below.
Failed to get a coordinate system from <filename>.prj; the file may be corrupt.
Starting with ArcGIS for Desktop 10.4, .prj files must include only projection parameters and information related to coordinate systems. Additional comments and the use of special characters in .prj files is no longer supported. Prj files containing text unrelated to the projection parameters render the .prj files corrupt.