Technically, Chinese money- the Yuan Renminbi- is still not internationally convertible and again technically can't be used outside of China. This is becoming something of a sham, but it's still the law.
Why do they care? When you convert RMB in China, you're exchanging them for a currency, such as the US Dollar ("Greenback"), the Australian Dollar ("Redback") or the Euro ("Brokeback") that can readily be transferred out of China...so therefore you are sending the wealth of the people out of the country.
This is considered "bad" in China, where they only like the hard currencies to be INbound.
Enforcement? Not much. The banks will exercise some control over international bank transfers, but not sure you'll have much problem unless you are sending out amounts large enough to be worth the trouble of looking up your records on a computer.
You can completely evade this enforcement by using a
black marketeer currency exchange consultant to exchange your money, and either using Western Union for some of the transfers or physically smuggling the cash out yourself. This path is of course risky and recommended only for seasoned China veterans...but personally I almost never used official routes for currency exchange once I'd been there a while.
I never really had enough money to challenge the official limits.
Indeed, for most teachers this is a bit of bureaucratic kerfuffle that has meaning only to the Chinese. If you're not sending out tons of money, don't worry about all this.