This Technical Specification defines the information content, structure and possible printed representations of the S18 ID-tag. This is an identifier for individual mail items which:
- is globally unique;
- can be applied to any item which is not already ID-tagged by any postal administration (or other issuer) which previously processed the item;
Whilst being generally applicable to domestic mail, the specification has been designed to allow the encoding of cross-border mail and to support its application in the automated processing of such mail.
A key purpose of the specification is to allow information about items to be exchanged between postal handling organisations. Such exchange will take place through the electronic communication of messages (e.g. EDI messages), or through the exchange of computer files.
The specification should be applied in all cases in which ID-tags are placed in area R1 on the reverse side of letter mail items of size up to and including C5.
Other forms of identification may be used in locations other than R1. Issuers using automated sorting systems are, however, strongly encouraged to identify items using this Technical Specification even when the items carry some means of identification which has been applied by another agency, outside of issuer control.