At present, the flight numbers of various domestic routes are composed of Arabic numerals in the thousands place. The first digit is the administration bureau code, the second digit is the code for the administration bureau where the destination airport belongs to, and the third and fourth digits are the sequence numbers for different routes of each administration bureau. Once you know the codes for the 9 administration bureaus (or airlines) under the Civil Aviation Administration of China (CAAC), you will understand how the flights are arranged. 1 represents Beijing Bureau, 2 represents Xi'an Bureau, 3 represents Guangzhou Bureau, 4 represents Chengdu Bureau, 5 represents Shanghai Bureau, 6 represents Shenyang Bureau, 7 represents Taiyuan Bureau, 8 represents Xiamen Bureau, and 9 represents Xinjiang Bureau. Take flight 6503/04 as an example: "6" indicates that it is an aircraft from the Shenyang Administration Bureau, "5" means it is flying to Shanghai, "03" is the sequence number for different routes flying to Shanghai at different times, and "04" is the return flight number for this trip.