When you place a “post only” order, you guarantee that your order will not immediately match with another order. This is useful if you want to ensure that your order does not immediately eat into someone else’s order, or if you want to avoid paying taker fees.
Your order will remain in the order book until it is matched, or until you cancel it.
If your “post only” order does immediately match with another order, it will be canceled. If this happens, you will see a message in the activity feed stating that your order was canceled.
You may also see this message if you have placed a “post only” limit buy order below the current market price, or a “post only” limit sell order above the current market price.
In other words, “post only” means that an order is placed on the order book, but is not matched with another order. The advantage of this is that it allows the user to avoid paying taker fees.