This International Standard specifies a data structure known as the SGML Document Interchange Format (SDIF). SDIF enables a document conforming to ISO 8879, which might be stored in several entities, to be packed intoa data stream for interchange in a manner that will permit the recipient to reconstitute the separate entities. SDIF also allows related documents to be included in the data stream, such as covering letters, transmittal forms, catalog cards, formatting procedures, or the "document profile" required by a document architecture.