Normally, if you open a file variable, of text type, without first assigning it a name, then the file variable will become associated with the the standard input stream or the standard output stream, depending on whether you are opening the file variable for reading or writing. In which case reading from the file variable will read from the standard input stream, and writing to the file variable will write to the standard output stream.
When this check box is checked and you open a file variable, of text type, without first assigning it a name, then the file variable will become associated with a temporary disk file and not the standard input stream or the standard output stream.
NOTE: Opening a file variable, that is not of text type, without first assigning it a name, will always associated the file variable with a temporary disk file, regardless of whether this check box is checked.