The Previous Day Equity with Loan Value (ELV) displayed in the Account Window is from a snapshot taken of the account's Securities segment's marginable Equity with Loan Value as of 16:15 EST the prior day. Only funds resting in the Securities segment are included in Available Funds which is used to calculate securities Buying Power. In a Margin account, the Available Funds number displayed in the Account Window is equal to Current ELV less the Current Total Initial Margin. However, if an account has been designated as a "Pattern Day Trader" then the Securities Available Funds formula will be the Lesser of either (Current Equity with Loan Value or the Previous end of Day Equity with Loan Value) minus the Current Initial Margin.
If a new order is rejected due to the post trade total Initial Margin exceeding the "Previous Day's ELV", then to determine the account's real-time Available Funds for new Securities orders you need to take the Previous Day's ELV and subtract the current Initial Margin. If that amount is negative, then the real-time Securities Available Funds is negative and you cannot enter a margin increasing order at that time. When Current ELV is higher than the Previous Day's ELV you will have the ability to trade securities with those extra funds after 16:15 EST (when today's Current ELV number will become the Previous Day's ELV number at the next end of day account snapshot).
Some examples of when an account that has been designated as a Pattern Day Trader will have its real-time Securities Buying Power limited to the formula including Previous Day's Equity with Loan Value less current Initial Margin would be when:
1) The account increased in value after the end of day snapshot was taken yesterday at 16:15 EST
2) If funds were swept back to the Securities segment after 16:15 EST yesterday
3) If a Reg T account sold some long stock options right after the 09:30 EST opening today
Remember that stock options have no loan value in a Reg T account. Option value is always included in the account's Net Liquidation Value but in this example the value of an open option position would not be included in yesterday's ELV when the account snapshot is taken at 16:15 EST. When you sell a stock option today the Cash received from the option sale is included in the current ELV, but Securities Buying Power will still be limited to the lesser number using Previous Day's ELV until a new account snapshot is taken at the end of today.