It can be sent with the document if it is a proprietary one or you can refer to a DTD if it is know by the partners of the exchange. Some DTDs are publicly available or standardized and are proposed as basic tools by the SGML editors. One of the main differences of the MARC structure and SGML is that the structures expressed in SGML can be very deep in the tree of the hierarchy compared to the flat structure of MARC data. SGML allows to describe the structure of a collection of documents and then the internal structure of the component items and you can reuse basic structure blocs at different level of the hierarchy. You can use theoretically the same tools for format encoding and for exploitation of the information at any level of the hierarchy or embedded bloc.
Specific users can add their own coding for specific usage.
Structure of the article frontmatter defined in the ISO 12083 article DTD Structure of the Authorgroup element defined in the ISO 12083 frontmatter article DTD Structure of the name defined as an entity that can be used everywhere in the whole document Characters coding In the declaration of the Document type you specify the basic character sets you will use in the document. Such a notation can be easily captured when keying with basic capabilities and transferred in data exchange including the networks we have today. They just have to be properly interpreted when printing or displaying or transcoding in specific applications. The SGML document is basically a text with beginning and ending tags marking up the logical elements or the attributes or the call for entities that can be textual information or images or video. The SGML tools also allow to view the SGML document with the tags hidden, using a style sheet for the layout.