There are many codes seen, because of many mail pieces to be sent in that country having a huge population.

No further info is found unfortunately. Those are issued by the China Postal Administration.

Among these:

RJ123456789CN – The recent experiences show that this is a Registered Airmail service, from China Outbound (the dispatch day) to Turkey Inbound (the arrival date) it takes around in 5 days! But when it comes to the total time, its not stable, because of the time past between the local post office sent and the time arrived at the Outbound in China, plus the time just after the Inbound arrival at your country until you have received the mail piece finally.

RF123456789CNThis is a Registered Air Parcel service.

Known other initials: RB, RD, RG, RH, RI, RK, RM