your features what you can choose
you want to transfer instantly and don't provide a upper limit of the conversion you want (you find a guess what your average conversion will be on the top page if you but the BTC amount into the field and push "BTC"
you want to transfer but want to get sure not to pay more then xxxx, so you provide a "limit" for the conversion and a time your limit will be canceled. The market maker tries to fullfill your order as much as possible and if something left and your order expiere the not converted part will got back to your BTC address
you want to transfer and you don't care about the price. What you care is not to pay xx% more then the feed price at a particular time. So you place an order with xx% above the feedprice. Only user can execute this prices if you place your relative order below our own market maker, but you car be sure if a sell happens you are the first who will get executed.
Hope this will help a little bit, too!