Post-only order
A post-only order is a limit order that is cancelled, rather than filled, if it would trade immediately. It only ever adds liquidity to the order book.
Because it always rests in the book first, it is always a maker order and pays the lower maker fee, never the taker fee. The cost: in a fast market it may be cancelled instead of filled. See maker vs taker fees.
Learn more: types of orders.