Based on the information in the manual, the ATX to PS/2 is some sort of converter that you plug in to the MALE end of the ATX PSU and plug into the PS/2 (mouse/keyboard port) of the motherboard.
This CPU board can support an advanced soft power switch function if an
ATX power supply is used. To enable the soft power switch function:
1. Take the specially designed ATX-to-PS/2 power cable out of NC-674’s
2. Connect the 3-pin plug of the cable to EXTATX (ATX feature connector).
3. Connect the power on/off button to PW/ON. (A momentary type of
button should be used).