Usually, it is US dollars. When settling, you need to return US dollars plus handling fees. This is how most domestic banks settle the money. As for whether to immediately convert money into U.S. dollars and then into RMB according to the exchange rate of the day when you spend money, or whether you should purchase foreign exchange and repay the money yourself, you can usually set it yourself. You see, if you exchange Hong Kong dollars for US dollars and then for RMB, you will pay more after changing a few lots. A better way is to directly apply for a multi-currency credit card, such as one that settles directly in Hong Kong dollars, which will save money. There are also some cashbacks for U.S. dollar purchases, which are also good. In short, credit cards are fully competitive, and each company has different policies. You can find the one that suits you best.