Ethfinex implements a maker-taker fee model.
Maker fees are paid when a trader adds liquidity to the order book. This is done by placing limit buy orders below the current price, or limit sell orders above the current price.
Taker fees are paid when a trader removes liquidity from the order book. This occurs when an an order is executed against an order on the order book.
Placing a market order incurs a taker fee, as the order executes immediately against the order book.
Placing a limit order incurs a maker fee, if executed, as the order adds liquidity to the order book.
If you place a hidden order, you will always pay the taker fee. If you place a limit order that matches a hidden order, you will always pay the maker fee.
For a detailed overview, check out the Ethfinex Fee Schedule.