First of all, single currency and dual currency refer to whether the functional currency of a credit card is one or two. To put it simply, China people use RMB for bookkeeping, and Americans use US dollars for bookkeeping. Different countries have different functional currencies, which is the difference.

Secondly, a single currency means that only one currency can be used for settlement. Dual-currency credit cards can naturally be settled in two currencies, but these two currencies are not arbitrary, but conditional. The most common dual-currency card is the RMB+USD card (there is not only the UnionPay logo on the card face, but also the logo of VISA or MasterCard, and the card number usually starts with 43 or 52). When using consumption, it will be charged in US dollars, but it will eventually be converted into RMB bills.

Assuming that it is a UnionPay +VISA card, you can only swipe your card on a POS machine with UnionPay logo or a POS machine with VISA logo. Otherwise you can't swipe your card.

If you swipe your card on a POS machine with the UnionPay logo, it is as simple as swiping your card in China. If you swipe your card on a POS machine with a VISA logo, you will eventually have to return the US dollars to the bank.