Check this if you need or want to try software-only Flow Control. This is often less reliable at high modem speeds than Hardware flow control. However, this setting may be useful for modems that implement Hardware flow control incorrectly. (If you have an external modem that does not work correctly with Hardware you might also have a faulty serial cable.)
If you use PC Access at a Board Office and you use the Direct Connection Method, you may have better results with XON/XOFF flow control.