See the reverse picture here: https://cdn.shopify.com/s/files/1/0174/1800/products/1912-00_1024x1024.jpg?v=1403270286
The second pin labelled "Vi2c" determines the signal voltage for the i2c communications. This allows you to power the unit with say a 5V supply, but set the logic level to 3v3 (ideal for the Raspberry Pi!).