As I find out, there is an abbreviation for each province in China, for example, 北京 is 京, 广东is 粤, 上海 is 沪, 广州 is 穗 and so on, but in many cases these three cities are also known as 北上广 rather than 京沪穗。
Another example is that: A CBA basketball match between Guangdong and Beijing teams is 粤京大战, according to sport reports in media.
The development plan of a western region is known as 川渝双城记 (if 川 is Sichuan, how can it be a city?)
The city of Nanjing is shortened to be 宁, but that is also the abbreviation of 宁夏, and the Railway Bureau of Nanning or 南宁铁路局 is called 宁铁 in short, but not 南铁 or 邕铁, as 邕 is the official short form of 南宁.
So what is the rule for that?