Default extensions depend on how the file is specified on the invocation line. A default extension is always placed on a file unless one is explicitly specified.
If an output filename consists of just a directory, it should always be terminated by a "\"; if not, it is treated as a file name. Thus, /L=txt\e outputs the listing to file txt\cat.lis (assuming cat.lib is the input file). /L=txt outputs the listing to txt.lis.