Hello.
That regulator boosts VBAT to 7.45V to power the Arduino through its VIN pin. Voltages above 7.45V are passed straight through the regulator, so you could use higher voltages like 8.4V. This voltage will show up on the Arduino VIN and on the Zumo reflectance sensor array, if you are using one. Note that the Zumo reflectance sensor array was intended to be powered at 7.45V; higher battery voltages will cause the IR LEDs to be brighter, which could affect the behavior of the sensor and could cause the emitters to prematurely burn out.
-Jon