When placing a limit order, you can specify the order attribute as Post-Only or IOC. If you choose Post-Only, your order will only be executed as a market maker. This means that if there are already orders in the order book that match your limit price requirements when you place the order, your order will be immediately cancelled.
IOC stands for Immediate-Or-Cancel. The function of an IOC order is that any portion of your order that is not immediately filled will be immediately cancelled by the system. This means that if there are no orders on the market (order book) that can be matched with your order, your order will be cancelled. If you choose an IOC order, your order will only be executed as a market taker.
IOC and Post-Only options are mutually exclusive. If you choose an IOC order, the Post-Only button will be disabled, and vice versa.