Buy button:

If switch is set as Market order then instruction to Buy specified amount of a chosen currency pair at the current bid price will be sent to a dealer.

If switch is set as GTC order then Pending order to Buy specified amount of a chosen currency pair at the specified price will be sent to a dealer and placed in TradingDesk Pro 5 system.

If you have pressed Quote Button and dealer have sent you prices for sell and buy, than pressing Buy button automuticaly results in making deal.