It's the position of both governments. The People's Republic of China (the communist-run one) claims territory over both the mainland and the island of Taiwan. The Republic of China (“Taiwan”) is basically the pre-communist government and claims territory over both the mainland and the island of Taiwan.
The presence of the US military on Taiwan has kept the PRC from curb-stomping Taiwan.
The presence of the US military on Taiwan has kept the PRC from curb-stomping Taiwan.