max 128 switch inputs and 128 on/off outputs
This is the choice for people doing projects where everything is on/off, like organ projects. The project was originally done to control a band organ. It is used by people who want to “midify” an organ console, a thing that has a lot of switches–the organ keys, stops, pistons, etc.–that need to be made to generate MIDI output. For this you need a Core plus one DIN board for every 32 inputs. To control the output of something like organ pipes where you need a lot of on/off signals that are switched by MIDI input, you use a Core plus one DOUT board for every 32 outputs. You can use both DIN and DOUT boards to a maximum of 128 inputs and 128 outputs, a total of 9 standard boards. Beyond that, you can link multiple Cores together, which is often needed for midifying an organ console which can easily have more than 128 input switches.
See also http://www.ucapps.de/midio128.html