Eswar Prasad, a former head of the International Monetary Fund’s China Division, said the administration was applying the label in an “arbitrary and clearly retaliatory manner.” In a report released in July, the I.M.F. also found that China’s currency was broadly where it should be.

“The currency manipulation charge against China is difficult to support on the basis of objective criteria,” Mr. Prasad said.

How much control does China have over its currency?

The United States and many other developed countries let the market determine the value of their currencies, and typically influence that value only indirectly. For example, when the Federal Reserve raises or lowers interest rates, it can strengthen or weaken the dollar.

China manages its currency more actively, though the market still plays a role. Officials set a daily benchmark exchange rate for the renminbi, but allow traders to push the value up or down within a set range. Officials then use that trading activity to help determine the next day’s exchange rate, though they disclose few details about how that process works.

Recently, those market forces have been pushing the value of the renminbi down, as a weaker Chinese economy and Mr. Trump’s tariffs encourage investors to sell the currency.

Brad Setser, a senior fellow for international economics at the Council on Foreign Relations, said China had been resisting market pressures on its currency for much of this year. China has been reluctant to have the value of the renminbi fall too far or too fast, for fear of sparking a mass sell-off.

So the Chinese government has turned to its vast foreign exchange reserves, accumulated through years of China’s exporting more products than it imported. Beijing has used those dollars to purchase renminbi and prop up its value, Mr. Setser said.