We've been asked to document the pin allocations that we use for the I2C connectors on our boards.
Here they are:
The Int line is used to signal an interrupt from those boards that can generate one, including the MCP23017 Port Expander Board.
The 3v3 line is powered by the Interface Boards' voltage regulator, and can provide a much higher current than the Pi's 3.3 volt supply.