Legally Taiwan is part of China.
Internationally Taiwan is treated as a renegade province of China, and China is considered to have sovereignty over Taiwan. However, for trade purposes it is treated very differently from China.
Does the tariff apply to Taiwan as well?
Most countries apply completely separate tariff, quota and import/export control schemes when dealing with Taiwan and China. However, most major Taiwanese companies now produce most of their goods in China, so the country of origin of something that appears to be from Taiwan many actually be China. Country of origin is complex, because most products are assembled from pieces made in various countries. There can be a complex procedure to work out where enough value was added to consider any particular country the country of origin. When the product has a prominent "Made in X" mark on it, that is usually the place where the final assembly occurred, which may not be considered the country of origin.