PostOnly is an optional feature available for limit orders.
When you use PostOnly to place a limit order, you can be sure that the order will be placed as a maker.
As a general rule, limit orders are not executed immediately after they are placed but are placed as a maker order in the orderbook. However, there is a time lag between the time an order is placed and the time it responds to the orderbook, so the (maker/taker) status of the order may change.
If you select PostOnly when placing a limit order, the order will not be placed as a taker, and only the order as a maker will be placed.
Currently, bitbank.cc charges 0.02% minus commission for orders executed by the makers, so if you use PostOnly, you will receive the maker's commission when the order is executed. In exchange for the immediacy of order execution, you will be able to trade at favorable trading commissions.
If you place a limit order with PostOnly, it will be cancelled if there are any opposing orders in the orderbook that will be partially executed. This is because PostOnly promises that the order will not be executed by the taker.
▼ Example
If a sell order has been already placed with a price of 1,000,000 yen and a quantity of 1 BTC, a limit buy order with a price of 1,000,000 yen and a quantity of 0.1 BTC is placed.
→ If a PostOnly order is placed, the order will be canceled.
If there is a sell order for 1 BTC at 1,000,000 yen as the minimum price on the sell orderbook, a limit buy order with a price of 990,000 yen and a quantity of 0.1 BTC is placed.
→ If a PostOnly order is placed, the order will be placed to the orderbook.
During trading modes other than Normal Mode ("Circuit Break Mode," "Preparation for Reopening Mode," "New Listing Mode," and "Indefinite Circuit Break Mode"), the PostOnly option is disabled due to the use of a different Taker/Maker decision method in Normal Mode.
Please refer to "How Trading Commissions are Determined" and "PostOnly Orders" in the explanation of the Circuit Breaker System for specifications by trading mode.
▼ Explanation of the Circuit Breaker System
https://bitbank.cc/docs/circuit-breaker-mode