When on the trade tab, you’ll notice two dropdown selections at the top left of your screen.
Exchange: Allows you to switch the context of the trade tab to another exchange. Switching the exchange allows you to see all public data related to that exchange including available markets, order books, trade history etc.
Account: Allows you to select an account you have linked from one of the supported exchanges. When you select an account, the exchange is switched automatically to show you the relevant public data, and all your balances, orders and transactions from the accounts are loaded so you can interact directly with that account as if you were on the exchange.