As the extended trading hours offered for stocks are generally not offered for options, there are periods during the day where a portfolio comprised of both stock and options will reflect a change in the stock price without a corresponding change to its option prices. As this can result in a Net Liquidation Value which would not likely exist were both products trading, IBKR computes a value referred to as Net Liquidation Uncertainty which attempts to reflect what the change in Net Liquidation Value would be if those options were trading.
This Net Liquidation Uncertainty balance is computed by determining the theoretical value of the option, taking into consideration the current value of the underlying stock. A positive value suggests a loss on the option portion of the portfolio that is not accounted for in the Net Liquidation Value. This computation is presented on the Account Window within the TWS.