you should check the manual, but usually:
. . . "RTTY mode" is used when you have an interface that does "true RTTY" or "mark/space keying".
For such an interface, the MARK signal is at one (DC) voltage, and the SPACE signal is at a different (DC) voltage.
The rig has circuitry that uses those voltages to shift the frequency of its carrier. The MARK/SPACE signal is applied to dedicated RTTY pins on the 13-pin "ACC" connector on the IC-718.
The signal transmitted is the same as the signal using AFSK, where the audio MARK and SPACE frequencies are used as SSB modulating signals. However, there are some technical reasons why "true RTTY" is preferred by some hams.
If you want to pursue this, there's an interface by W3YY:http://www.eham.net/reviews/detail/8519
And there's probably more information at: