A structured editor is a kind of intelligent word-processor. It can use information extracted from a processed DTD to prompt the user with information about which elements are required at different points in a document as the document is being created. It can also greatly simplify the task of preparing a document, for example by inserting tags automatically.
A formatter operates on a tagged document instance to produce a printed form of it. Many typographic distinctions, such as the use of particular typefaces or sizes, are intimately related to structural distinctions, and formatters can thus usefully take advantage of descriptive markup. It is also possible to define the tagging structure expected by a formatting program in SGML terms, as a concurrent document structure.
Text-oriented database management systems typically use inverted file indexes to point into documents, or subdivisions of them. A search can be made for an occurrence of some word or word pattern within a document or within a subdivision of one. Meaningful subdivisions of input documents will of course be closely related to the subdivisions specified using descriptive markup. It is thus simple for textual database systems to take advantage of SGML-tagged documents. Much research work is also currently going into ways of extending the capabilities of existing (non-text) database systems to take advantage of the structuring information made explicit by SGML markup.
Hypertext systems improve on other methods of handling text by supporting associative links within and across documents. Again, the basic building block needed for such systems is also a basic building block of SGML markup: the ability to identify and to link together individual document elements comes free as a part of the SGML way of doing things. By tagging links explicitly, rather than using proprietary software, developers of hypertexts can be sure that the resources they create will continue to be useful. To load an SGML document into a hypertext system requires only a processor which can correctly interpret SGML tags such as those discussed in chapter 14: Linking, Segmentation, and Alignment.