Order types, stops, post only, partial fills and the gap between the price you saw and the price you got.
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Traders who optimise their fee tier to the last decimal often ignore a cost several times larger, because nobody sends them an invoice for it. Slippage is the gap between the price you decided on and the price you got. It has causes you can name, effects you can measure, and a component you can genuinely reduce.
Liquidity is used as though it named a single quantity that a market has more or less of. It names three, they are measured differently, they fail at different moments, and a market that scores well on the one you looked at can be the one that costs you the most.
Most traders pick an order type by habit and discover its price afterwards, in the fill. Every type is the same trade in disguise: you are choosing between certainty of execution and certainty of price, and you cannot have both. What follows is what each choice costs, and where the cost hides.