TradeStation Help
Intraday Buying Power (Day Trading Rule 431/Rule 2520) is what truly limits the ability for you to place an order. If your Overnight Buying Power is less than their Intraday Buying Power, and an order is placed which exceeds only the Overnight Buying Power, the Buying Power Alert window will appear to notify you that you may need to close that position before the end of the day or they may incur a Regulation T Call.
Under no conditions you will be allowed to place an order that will exceed their real-time Intraday Buying Power, the order will be rejected by the trade server.