Clients can direct non-marketable U.S. stock orders to the IBKR Alternative Trading System (or ATS) to add liquidity. Orders directed to IBKR ATS are automatically tagged as "not held" orders, and are posted in IBKR's order book where incoming SmartRouted orders from other IBKR customers are eligible to take liquidity by trading against them.
Find out more in the IBKR ATS order page.