What is remsize in volumefilling sample code?

[TigerAndy]

Sample code: https://github.com/mementum/backtrader/blob/master/samples/volumefilling/volumefilling.py

def notify_order(self, order):
        print('-- NOTIFY ORDER BEGIN')
        print(order)
        print('-- NOTIFY ORDER END')
        print('-- ORDER REMSIZE:', order.executed.remsize)

        if order.status == order.Completed:
            print('++ ORDER COMPLETED at data.len:', len(order.data))
            self.doop = -self.p.opbreak