The AIU SW(itch) ports pulse on for exactly 1/2 second when you press the Straight or Diverge button on the remote. So these require an external latching DPDT relay. See this post showing various ways to do this for a Tortoise switch machine but from what I understand, your Switchmaster is similar.
Tortoise provides a useful summary of hookup options:
You can use the AIU ACC(essory) ports if you supply a positive and negative DC supply...and let the ACC port choose one or the other. This means you do not need an external relay per switch-machine. As the diagram suggests you can use a pair of DC-output wall-warts, or use a pair of diodes to supply pulsing positive or negative DC. I am not familiar with Switchmaster but if like Tortoise it should be no problem to accept this pulsing DC.
If you use the ACC port (instead of SW port), obviously the DCS remote button won't have the icons for Straight-Diverge. Also, with an ACC port when you initially power up the system, the AIU always resets to the OFF position. OTOH, if using an external latching relay on an SW port, the relay has mechanical-memory and remembers its last position. So you may choose to record/playback a sequence of commands to set up the starting positions.
It appears the Switchmaster has a 3V DC operating voltage. They apparently provide a resistor to operate the machines at higher voltages (like 12V DC). Once you settle on the general approach, if you need a more detailed diagram showing where the resistor goes (if needed), or exactly which terminals on the AIU go to where, just ask. It would be helpful to know how many switches you have and approximate distances.
Finally, in the linked thread, the OP there had to use the AIU SW ports because he had the "must have" requirement of dual control. In other words, he wants to control the switch-machines from either the DCS-remote OR pushbuttons on a control panel. While I suppose anything can be done given time and money, but if you have this dual-control requirement, the ACC method becomes more challenging.
