The pins in the front expansion area of the Zumo Shield are shown in the following diagram:
This diagram is also available as a downloadable PDF: Zumo Shield front expansion pinout (552k pdf).
The front expansion makes available digital pins 2, 4, 5, and 11 and analog pins A0 through A5. It also provides access to the two I²C pins (SDA and SCL). However, please note that the I²C pins are not independent pins; they are respectively duplicates of analog pins A4 and A5 on the Uno R3 and digital pins 2 and 3 on the Leonardo and A-Star 32U4 Prime. Typically, you will only be able to use these pins for either I²C communication or general I/O, not both. Additionally, pin A1 is used to monitor the battery voltage if you install the battery monitor jumper.
Please note that only components and connectors in the of pins can extend below the shield; the fourth row covers the chassis and is only suitable for components mounted above the shield.
If you use an Arduino Uno R2 or an older Arduino, which lack separate I²C pins, the SDA and SCL pins on the Zumo Shield will not be connected to anything. To use an I²C device on those pins, you can connect SDA to A4 and SCL to A5 yourself by bridging across those two sets of pins in the front expansion area. Section 3.c further explains the I²C lines and the jumpers connecting them to the on-board compass module.
Depending on the Arduino model, digital pin 3 or 6 is used to control the buzzer if you install the buzzer control jumper. If you are using an Uno, pin 6 will be available for general-purpose I/O. If you are using a Leonardo or A-Star, pin 3 will be available if you are not using I²C. These pins are not accessible via the front expansion, but they can be accessed from other points on the shield and used for interfacing with additional electronics if free. Additionally, digital pin 12 can be used for interfacing with many types of additional electronics, especially if you are not using the shield’s user pushbutton. Pin 12 is completely free when the pushbutton is in its default, unpressed state, and it is pulled low through a 1k resistor when the pushbutton is pressed.