Finally a Full Analysis: Renminbi vs. Yuan

When it comes to renminbi vs. yuan, Tyler Cowen ends the mystery, when in China use “kuai'":

The style guide of The Economist magazine, after explaining the difference between the two terms, leaves no ambiguity about what its reporters should use: “Renminbi, which means the people’s currency, is the description of the yuan, as sterling is the description of the pound.  Use yuan.”  The Financial Times favors the use of renminbi over yuan by a six-to-one ratio.  But Financial Times reporters seem to believe its readers are sophisticated enough to be able to shift back and forth between the two terms without further explanation.

That is from new and useful Gaining Currency: The Rise of the Renminbi, by Eswar S. Prasad.

And here is the BBC:

“Renminbi” is the official name of the currency introduced by the Communist People’s Republic of China at the time of its foundation in 1949. It means “the people’s currency”.

“Yuan” is the name of a unit of the renminbi currency. Something may cost one yuan or 10 yuan. It would not be correct to say that it cost 10 renminbi.

I did not know this:

The word “yuan” goes back further than “renminbi”. It is the Chinese word for dollar – the silver coin, mostly minted in the Spanish empire, used by foreign merchants in China for some four centuries.

If you wish to pursue it further:

As it happens, Chinese people rarely talk about renminbi or yuan.

The word they use is “kuai”, which literally means “piece”, and is the word used historically for coins made of silver or copper.