CEX.IO has a maker/taker fee schedule in place.
A maker order adds liquidity to the market. Such an order cannot be executed right away, because there is no matching order available. In this way, this order increases the volume currently available on the market, and so, increases liquidity. There is 0% fee for maker orders.
A taker order takes liquidity from the market. This is an order that can be instantly matched with another one, already available on the market. So, the order is completed and its volume is removed from the market, thus taking liquidity from it. There is 0.2% fee for taker orders.
Part of your order can be taker, if it is instantly matched with a smaller volume order already available on the market. Taker fee will be applied to this partial execution. The incomplete remainder will be considered maker order, as it will add liquidity to the market, and maker fee will apply accordingly.
How is the fee calculated?
A taker fee is reserved from your account at the moment of order placement. This is done, because there is no possibility for the system to know, whether the order will be maker or taker before it is added to the order book. If the order is added as maker - the reserved fee is immediately returned to users account. If the order is partially completed upon placement, taker fee for the partial completion is taken, and the rest is returned. If the order is wholly completed as taker, the full amount of reserved fee is taken from it. Trade fee is always rounded up.