I emailed arduino support and had this very helpful reply...
Moreover, regarding the connectors, there are the J4 and J7. The first one [J7] is an additional I2C Port, meant as an extension of the I2C bus. It's a small form factor 5-pin connector with 1.0mm pitch. The mechanical details of the connector can be found in the connector datasheet. The I2C port in addition to the SDA and SCL signals includes the GND and +5V power rails and a digital pin that might be useful when designing an expansion.