Dazzer is correct. In China you pay tax only on declared income. However, China tax law is quite clear; you must declare all income.
I think a safe bet is that governments determine what of your income is taxable. And, I am sure governments fully expect all income, as defined by the government, and all tax due on that income is paid. Governments, all of them I am sure, will take measures to insure compliance with the tax laws and will attempt to collect, along with penalties and fines, all tax due.
The government defines what activities produce income. Any remuneration derived from those activities is taxable. The key factor in understanding taxation is to realize you owe at least an accounting to the government for all income. The government requires you to 1) report all income, and, pay all taxes due. Failure to comply with tax laws laws can lead to various unpleasant actions by the government.
I use "government" as the above are pretty much universal in what ever country you earn your money and, in the case of Americans, also applies to US taxes where ever the income is earned. Each of us needs to be aware of China's IIT and the laws of whatever country we are citizens of.
Be aware China tax enforcement, for everyone, has been hit or miss, perhaps lax, and the Chinese government knows this. For several years, China has been developing a tax enforcement system. Essentially, it is a huge database focused on collecting information regarding payments, receipts, and personal data. This multiyear effort is being built and beta tested on a limited target group: expats. All employers report up to tax bureaus and the tax bureaus report to the provincial level and eventually there will be a national database.
Additionally, income earners are required to file monthly reports on income reported and NOT reported by employers. Part time or side jobs and occasional work is to be included in this self filed tax return. Under the IIT, this income is taxed at different rates, and calculated differently, than income earned under contract at a primary employer. The IIT refers to this type of income as "personal services."
China is "nationalizing" IIT. This will standardize and unify application of the tax law, consistent with the national database program, as local tax bureaus come under the direction of the State Administration of Taxation (SAT). In the past, each local Tax Bureau could determine how to apply tax laws and calculate tax.