More than 20 countries around the world have established air defence identification zones, or ADIZs, including the US, Japan, South Korea and, since November, China.
Aircraft flying into another country’s ADIZ are required to identify themselves and provide flight data.
China said aircraft are required to give notice of their intention to fly through the zone, or face possible retaliation.
ADIZs can expand well beyond sovereign airspace, and even beyond a state’s exclusive maritime economic zone.
China’s newly declared ADIZ overlaps the ADIZs of Japan, South Korea and Taiwan. Japan enlarged its own air defence zone in 2010.
More controversially, China’s ADIZ covers islands that are administered by Japan as the Senkaku islands, but claimed by China and Taiwan as Diaoyu and Tiaoyutai respectively.
The area laid out by Beijing also includes a formation known as Socotra Rock, which is disputed between China, where it is known as Suyan, and South Korea, which calls it Ieodo.