The Graphics Master includes a quad UART in additition to the three StrongARM serial ports. The quad UART ports are known as Serial A, B, C and D, and the signals come out header J20.
The drawing indicates how to build a cable that places the Serial A and Serial B signals on DB9 plugs suitable for connection to other RS-232 devices. Serial C and D are configured for 3.3V CMOS operation on standard GMaster systems; since there's no standard for logic-level serial connections, they are left unterminated.