Today typically a Business needs to hold accounts in more than one currency. Banks in certain countries are offering what is called a dual currency account. It is essentially 2 accounts with same account number but different currency. So One can have an account number say 123456 and have it in say AUD and USD.
So the balance will always show as X AUD and Y USD.
If you deposit funds [electronic, check or cash] in USD; your USD balance goes up.
Likewise at the time of withdrawal you have to specify what currency you are withdrawing.
Interest rates are calculated at different percentage for different currencies.
So in a nutshell it would like operating 2 accounts, with the advantage of remembering only one account number. Designate a particular currency as default currency. So if you don't quote a currency along with the account number, it would be treated as default currency. Otherwise you always quote the account number and currency. Of-course bundled with other services like free Fx Advice etc it makes the entire proposition very attractive.
If you have AUD 100 and USD 100, if you try and withdraw USD 110, it will not be allowed; Unless you also sign up for a auto sweep conversion.
If you deposit a GBP check into the account, by default it would get converted into AUD [assuming AUD is the default currency]