The Post Expiry Excess is a projected value of your Excess Equity after taking into consideration the effect of option contracts expiring over the next 3 days. Here, maintenance margin is projected on the assumption that expiring option contracts which are in-the-money will be exercised and delivery taken and contracts that are out-of-the-money will expire worthless.
The delivery (i.e., purchase or sale) of stock associated with option exercise may significantly increase the margin requirement and the Post Expiry Excess is intended to provide clients with an estimate of the change so that they may make the necessary deposits and/or position adjustments to remain margin complaint.
Note that IBKR reserves the right to take protective action to minimize the likelihood of an account being non-compliant based upon option exercise projections. These actions may include positions liquidations and lapsing in-the-money long options. Liquidations generally start 2 hours prior to expiration and take into consideration the volatility of the underlying stock and the time remaining until expiration when projecting the likelihood of the option falling in-the-money. See KB1767 for additional details.