Each type of file created by WordSmith Tools has its own default filename extension (e.g. .LST, .KWS) and its own internal structure. If you have another file with the same extension produced by another program, this will not be compatible. It would not be sensible to rename a .CNC file to .TXT, or vice-versa! WordSmith has detected that the file you're calling up wasn't produced as a comparison file by WordList.