Motorola RAZR V3c

Guys, I just came back from upgrading my raza to 4 and I bought an extra battery and the moto. battery charger, but they kid threw out the box's... I see that you can snap out where the battery goes, but what is the space under the battery? Is it just for a bigger battery? Like I said he threw out the box w/the instructions so I don't know what it is... any help would be great...