Descent fuel tanks
My experiance is with 737's but I can't imagine this is different on the 777. Fuel is normally used first from the center tank so descents would normally be using wing tank fuel. This is because weight in the wings is more easily supported from a structural point of view (this is why there is a concept of maximum zero fuel weight on transport aircraft). In the 737, the center tank fuel pumps operate at a slightly higher pressure so that the center tank will empty first even if all tanks are online.