1. The only info I could find is this:
RA–RZ Letter post registered; the use of RZ requires bilateral agreement

2. I found this also:
Rules for assigning a service indicator to a mail item
Rules for assigning service indicators are governed by the UPU Regulations.
EXAMPLE In the case of letter post, with many ranges defined, the following applies when assigning a service indicator to a
letter post item:
– if a letter is insured, then its service indicator is in the range VA–VZ, regardless of the other characteristics of the item;
– if not, if it is registered, its service indicator is in the range RA–RZ, regardless of the other characteristics of the item;
– if not, then:
if it is express, its service indicator is in the range LA–LZ, or
if it is an M bag, its service indicator is in the range MA–MZ, or
if it is an IBRS, its service indicator is in the range QA–QM.
If it is not any of the above and if it is subject to customs control (bearing a CN 22 or CN 23), its service indicator is in
the range UA–UZ.

Hope this help.