China pretty much made claim to all of the waters it's claim extends as far as the shore lines of Malaysia.
China also extends it's claim over waters outside of the immediate territorial waters by building artificial islands so far only within the 200 n/M of their exclusive economic zone which it claims extends it's coastal waters even further (this isn't exactly the case for this specific island (it's land mass was artificially extended, and a large harbor was built) but an important background point for the entire dispute).
So far countries have refrained from militarizing the islands too much sure they might post a couple of sailors here and there but this is a long rang air defense system capable of shooting down aircraft as far as 400km away, this is basically an S-300/400 [https://en.wikipedia.org/wiki/S-400_(missile)] "copy" with a much more advanced radar and better interceptors by all counts.
Chinese source: http://blog.sina.com.cn/s/blog_49e4b637010007hi.html I don't speak Chinese but it seems to translate rather well.
The original HQ-9 interceptor is stated to have 200km slant range, just as the Russian missile it was faithfully copied from.
And yes I said that the system initially was deployed with a range of about 200KM but they have made improvements since: https://en.wikipedia.org/wiki/HQ-9#Variants
Most SAM's have different profiles and interceptors that cover targets with different capabilities this quite often limits their range, you can argue about the exact range and operational ceiling for the interceptors in question all day long it still doesn't change the fact that China has deployed one of the most sophisticated SAM's currently in operation on and island that they might not have full legal claim for which can threaten quite a big chunk of the air traffic over the south china sea.