To be honest, so far the trading works out ok.
I haven't figured out how the limit function works when closing positions tho.
But overall using the trading engine is pretty simple and smooth.
Thanks asche for speaking up.
In short, a limit order will be added to the market book and await a taker to fill it. It has a big chance not getting filled, but charges 0 commission fees on BaseFEX. It is like you open a store to sell things, but only get a deal when a customer comes to your store. Until a deal is made, the order remains there.
A market order is like a customer who wants to get a deal in someone else's store - he takes things away.