Post-only orders are an additional option that can be used with the limit order feature. It ensures that the limit order is placed into the order book but will not be executed in the market. Post-only will become a maker in the order book, adding liquidity to the market.
Post-only users are only charged maker fees, not taker fees, when the order is executed. In addition, the system will automatically cancel the limit order if it is detected that the limit order will be completed immediately after the order is placed.
For example:
Aldi placed a buy limit order on UNI worth IDR 5 million with a target purchase price of IDR 80,000. The best ask price in the order book at that time was IDR 80,150, but it immediately changed to IDR 79,700 once the limit order was placed.
Without post-only, the system will immediately execute the limit order as a market order. The order will be filled from the best ask price of Rp 79,700 to Rp 80,000. Aldi also has to pay taker fees, even though he hopes for cheaper trading costs (only pay maker fees).
Meanwhile, with post-only, the system will immediately cancel automatically because the ask price has changed to a better price (Rp 79,700) from the limit order price placed (Rp 80,000). The system cannot put the limit order in the order book. As a result, Aldi can avoid paying a more expensive trading fee (not subject to taker fees).