Application Identifier (AI) 253 specifies a Global Document Type Identifier (GDTI). A GDTI is used to identify a document type, with an optional serial number. The GS1 Company Prefix forms the first digits of the GDTI and ensures that the number is globally unique. The structure and content of the GDTI are determined by the owner of the GS1 Company Prefix and are used to uniquely identify each document type. The digit preceding the optional serial number is a check digit.
The optional serial number is assigned to an individual document for the entire lifetime of that document. When the serial number is combined with the GDTI, the complete number uniquely identifies the individual document.
The structure of the serial number is determined by the document owner.