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Physically, the document is composed of units called entities.
An entity may refer to other entities to cause their inclusion in the document. Logically, the document is composed of declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup.
Tags can be replaced with delimiter strings, for a terser markup, via the SHORTREF feature.
This markup style is now associated with wiki markup, e.g.
Although the markup norm is using angle brackets as start- and end- tag delimiters in an SGML document (per the standard-defined reference concrete syntax), it is possible to use other characters—provided a suitable concrete syntax is defined in the document's SGML declaration. (NOTE: A concrete syntax might change this rule via the NAMECASE NAMING declarations).
SGML has features for reducing the number of characters required to mark up a document, which must be enabled in the SGML Declaration.
SGML also was extensively applied by the military, and the aerospace, technical reference, and industrial publishing industries.
(ISO 8879 A.1) An SGML document may have three parts: An SGML document may be composed from many entities (discrete pieces of text).
However XML's well-formedness rules cannot support Wiki-like languages, leaving them unstandardized and difficult to integrate with non-text information systems.
The usual (default) SGML concrete syntax resembles this example, which is the default HTML concrete syntax: SGML provides an abstract syntax that can be implemented in many different types of concrete syntax.
Reference-free reflects the HTML requirement that entity references are for special characters and do not contain markup.
SGML validity commentary, especially commentary that was made before 1997 or that is unaware of SGML (ENR WWW), covers type-validity only.
A conforming SGML document must be either a type-valid SGML document, a tag-valid SGML document, or both.