I studied the International mail system by sending International Registered Mail, and here is what I found.
First, mail was accepted at the post office. The post office clerk will choose the mail type (Registered postcard, registered mail, registered printed item) and scan the barcode, select the destination in the system and choose the transport method(Surface, SAL, Air) and additional services/remark(returned mail/transit mail/fragile mail/A.R.) Registered printed item requires an ID card to accept.
After the clerk press “calculate price” button, the system will say whether its data shows this route is workable and show the price. If the system thinks that it is not workable, it will say "this business is not available”. After price calculated, customer buy stamps, print postage label or use their own stamps to pay the fee, and the clerk will cancel the stamp and press the “acceptance” button.
It is the time when this message will appear in the postal inquiry system.
It says Acceptance at [Hexi Xincheng Post Office] in Nanjing.
If you are sending items to some countries like UK, Japan and Swiss. Their system will also receive the information and can be seen in their system and UPU too.
Then, the clerk will print out a receipt or a list depending on the amount of item you are sending.
The list
The receipt
After that, the clerk will put it into a cart and wait for truck to pick up. The truck arrives at 14:00 and 17:00(GMT+8) every day. After the truck arrives, the clerk will press dispatch button and scan all barcode again to print out a waybill put together with the mail, and they will all be packed and sent.
This message will appear at the same time. Saying the mail is leaving post office and heading to mail hub.
International mail will be separated into 4 packs(Cecogramme Registered, Cecogramme non-registered, Registered mail and non-registered mail) and sent to the Sorting centre(In some cities, they will be sent directly to International Mail centre) for redirecting.
Information says leaving Nanjing Hub left for Nanjing International Mail centre.
If the package is sending to countries like Germany, Italy and Swiss. They can form a mail pack directly at Nanjing. Which requires a custom check.
[Nanjing International Processing Center], Exported
At this moment, the info will sometimes be available at destination post.
Example: Swiss
At this moment, this object can be defined as an item left the border and will be moved for further transport. It will also contain a mail pack barcode in forms like CNNKGA(Export office code:NJGJCLZX-NANJING)CHZRHB(Destination office code:Zürich Briefzentrum International-ZURICH 1)XXXX11111111111111
In other conditions, the mail will be separated according to destination country and shipping methods and transported to different export office.
SAL mail will all be sent to Beijing.
Surface mail will be sent to Shanghai and Guangzhou in most conditions. If it requires ground transport, it will be sent to Urumuqi
Air mail to Japan, USA , Belarus, Turkey and some SE Asian countries will be sent to Guangzhou
Air mail to European countries and some Asian countries will be sent to Shanghai
Air mail to other places will be sent to Beijing
Russian mail will all be sent to Zhengzhou at last