When this button is selected Fetch is in Binary mode.  In Binary mode Fetch will treat all files as binary files, no matter what the file name is.  This is the right mode if you know you are getting a binary file but Fetch does not recognize it as such.

When this button is selected Fetch is in Text mode.  In Text mode Fetch will treat all files as text files, no matter what the file name is.  This is the right mode if you know you are getting a text file but its name makes it appear to be a binary file.

When this button is selected Fetch is in Automatic mode.  In Automatic mode Fetch will try to determine whether a file is text, BinHex, or binary by examining its file name, using the Suffix Mapping list.