請教關於“trading buying power”的含義，先謝了

如果隻做股票，下麵問題（Yahoo上一哥兒們問的，也是俺想了解的）答案是？

is the 4x day trading buying power the maximum value of initial security investment on the day, or is it the maximum position that can be opened together and go back up as positions are closed throughout the day, so that i will be allowed to trade in other securities if opportunity present itself.

比如賬戶有五萬現金，每天隻做MSFT，trading buying power應該是20萬。這20萬是指每單比交易的上線呢，還是指每天交易總額的上限？