In forex (fx) trading, the transaction currency (also referred to as base currency) is the first currency listed in the currency pair.  When creating a forex order with Interactive Brokers, the order quantity is entered in terms of the transaction currency.

Example:

An order to buy 100,000 EUR.USD @ 1.353 would result in the following;

buy 100,000 EUR

sell 135,300 USD (computed as 100,000 X 1.353)

see settlement currency