请教关于“trading buying power”的含义，先谢了

如果只做股票，下面问题（Yahoo上一哥儿们问的，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.

比如账户有五万现金，每天只做MSFT，trading buying power应该是20万。这20万是指每单比交易的上线呢，还是指每天交易总额的上限？