Thanks for contacting the Community for support. I'd be glad to offer my assistance on using Multi-currency in your QuickBooks Online account.
According to the overview, "The Multi-currency feature is used by small business owners who have customers, vendors, vendors, or bank accounts that use foreign currencies, or currencies that differ from their Home currency." It's important to know that once Multi-currency is turned on, it can't be turned off. This article offers additional details on how Multi-currency works: https://quickbooks.intuit.com/community/Getting-Started/How-Multi-currency-works/m-p/186157.