Heres a pic from another pinsider showing what came with his batsignal topper that came with the node boards.
Here how I interpreted what was going on. I could be totally wrong though as I was just going by what I could figure out from the pics.
There were three boards. First board (top of pic) was right under the projector and it had a relay that when toggled it could turn the projector on and off. I think it needs the relay because the lamp Stern uses need 48volts so the relay let's you control the 48v via a lower level 5v signal.
The second board (middle of pic) was an intermediate driver board that recieved commands as to when to turn from the node board and sent signal to some drive transistors.
The third board (bottom of pic) was the node board. This node boards interface with the CPU via a cat5 cable. The communications on the cat5 cable are what contains the signals to turn lights, motors, relays, etc on or off during game play. The node boards job is to decode those signals and then output simple on/off trigger signals to the driver boards.
I think the relay board, the driver board and the node board could have be combined into one board. I'm assuming Stern used 3 boards because they had those boards readily avail and it wasn't work the effort to make a custom one just for the topper signal.
Maybe the latest version of the stern batsignal has combined all these into one?