The pins defines are all in the firmware code defines.h:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/defines.h#L388
assuming you have OpenSprinkler version 3.2 (previous versions use different pins, and they are all defined in the file as well). IO_EXP + ? means it’s pin number ? on the PCA9555 IO expander.