The trading volume of opened position and closed position will be computing separately. For example, if you opened a position of 10,000 GBP/USD and closed in this month, total trading volume of 20,000 will be computed in this month. If you opened a position of 10,000 GBP/USD and keep it, the trading volume of 10,000 will be computed for this month and the trading volume of the close position will be computed for the month you close the position afterward.