I've never understood.
So: renminbi is the name of China’s currency; but yuan is the denomination of bills, the unit in which prices are measured, etc.. The closest parallel I can think of is Britain’s currency, which is sterling, but whose unit is the pound.
In the case of Britain, however, everyone is easy on talking about the pound’s value, the pound’s exchange rate, and so on; if you talk about sterling’s value, most non-Britons will have no idea what you’re talking about. But for whatever reason, using yuan in the same way draws disapproval.
But here’s the thing: talking about the number of renminbi per dollar is also, as I understand it, wrong — as wrong as talking about the number of sterling per dollar. Renminbi is the currency, but not a unit of the currency. [More]
And...I still don't.
And how do you pronounce it, fer cryin' out loud?