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. (...)