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; NOTE 1 The S18 ID-tag provides a standard means of ID-tagging which can be applied on a world-wide basis, allowing inter-administration mail items to be encoded without risk of disruption of the automated system of the delivery post. It may be applied to any size of item. can be read, with a high degree of reliability, by any postal handling organisation possessing appropriate equipment. NOTE 2 ID-tags are encoded on items using a bar code symbology. As with any other form of bar code, poor quality printing, ink smudging, damage to the item, etc., can result in read errors. The S18 ID-tag encoding specifications incorporate an error protection mechanism to allow detection and correction of a large proportion of such errors. The S18 ID-tag defined in this Technical Specification may be placed on items so that, in subsequent processing, individual items can be recognised and associated with computer-based information relating to the item concerned. NOTE 3 Items need not be ID-tagged if this is not required for processing purposes, though it is anticipated that the use made of ID-tagging will increase. Examples of ID-tag applications are given in Clause 7. 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. NOTE 4 UPU regulations prevent the encoding of information on the bottom 15 mm strip on the front of international letter mail. This Technical Specification has been designed to avoid the possibility of incompatibilities between ID-tag implementations and explicitly allows and encourages the application of S18 ID-tags, in area R1, on the reverse side of items. This will enable exchange of data about the items and use of these data for automated processing. For example, the origin post might send an encoded or video image electronic copy of item delivery address blocks to the delivery post, enabling the latter to more efficiently process received items. Similarly, the delivery post might return time-of-processing data to the origin post, enabling it to monitor delivery service quality more closely. 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. NOTE 5 The detailed requirements, content and usage procedures for these messages or files are/will be defined in other standards, which use the S18 ID-tag as the means of associating information with the item concerned.