In fact it’s not control board. It contains three MCP23S17 modules intended to be connected to MCP’s Arduino board.
MCP23S17 allows to connect up to 8 devices on single SPI line. Each device has 3 pins to set it’s address. Each EFIS can be configured as captain’s or 1st officer’s with DIP switches, that are used to set address of MCP23S17. That’s how it works.
Board installed on the EFIS module: