This button enables you to open a new window, displaying the text filename from which your current data comes. You can edit these names if necessary (e.g. if the text files have been moved or renamed.) To do so, choose Replace ().
Afterwards, if you save the results, the information will be permanently recorded.
In the case of key word lists, the data comes from a word list. If the word list was based on just one text file, you'll see the text file name, but if on more than one, you'll see the name of the word list file itself: to see the original text file names, you could open up the word list and press the filenames button in that.
See also: finding source files.