Once logged in, the base currency drop down can be found in the top right hand side of your tool bar.
This option allows you to select what currency you will manage your account and trade with. It is common practise to trade assets based off other assets like Bitcoin, Ethereum or US Dollars.
This is useful if you have a preference to trade the markets based off their USD value. You can deposit AUD which will be converted to USD automatically. All trades and balances shown in the app will now be in USD.
If you're a Satoshi trader, same principal applies. Simply fund your account with Bitcoin either via deposits or purchases using Fiat and then view your balances and trade all your favourite assets with Bitcoin.