When you sign up for a trading account with Stake, you get an AUD wallet, which is a Macquarie Bank cash management account (CMA). A CMA is a bank account that is free to open, earns you interest and gets you access to seamless FX transfers with OFX.
With a CMA, you give Stake authority, once you've requested a transfer, to arrange for the transfer between your AUD wallet and your US trading account via OFX.
A copy of the Product Information Statement for the CMA is provided by Macquarie here.
As your CMA is your own bank account, you can use it for any purpose it allows. If you plan to use it outside of the Stake platform, a list of its fees can be found here.
For more information from Macquarie regarding the CMA, please see Macquarie's Further Information Guide.