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.
Limit orders allow you to limit the price you pay, but the order may not be executed immediately / completely. The remaining order value will be added to the orderbook.
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.