Basket Trader lets you trade in multiple instruments at once at the market price.
To trade using Basket Trader, you have to add instruments by dragging them into it. You can then specify the quantity for each one of the instruments and the side of the trade for each specific instrument (buy or sell). You can set the multiplier for the basket overall volume.
To perform a trade, click Buy or Sell. Please note that if you choose to Sell any one of the instruments and then click Sell for the entire basket, the instrument will be bought, not sold. Take this into account when managing your Basket Trader.
As to the Quantity and Multiplier buttons, they both change the displayed cost of the basket. If they both are inactive, the basket price is shown for 1 item of each asset. Otherwise, the sum is affected by the activated parameter. Please note that the Quantity and Multiplier buttons do not affect the actual sum of your order.