BL3P offers two types of orders: market orders and limit orders:
With a market order you instantly buy or sell bitcoins based on the current bids and asks, no matter what the price will be. Therefore the actual price can differ significantly from the indication. Be cautious with large market orders: especially when the orderbook is thin, the price you pay or get can be disadvantageous. It is probably a good idea to split up your order and/or use a limit price.
Limit orders allow you to specify a maximum price to pay, or a minimum price to receive for your coins. BL3P will directly match any available bids or asks with an equal or better price. The remainder of your order (or the complete order) will be placed into the orderbook. If and when the remainder will be executed can never be known for sure.
For both market orders and limit orders, you can specify exactly how many bitcoins you want to buy or sell, or how many euros you want to earn or spend. The exact type of order will be based on the field you changed last.
Note that when entering a euro amount to earn or spend, small rounding differences may occur in the price and bitcoin amount.