!!! Overview
[{$pagename}] typically refers to the description or [Schema] of a type of [XML] document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by [XML] itself. 

These constraints are generally expressed using some combination of grammatical rules governing the order of elements, Boolean predicates that the content must satisfy, data types governing the content of elements and attributes, and more specialized rules such as uniqueness and referential integrity constraints.

[{$pagename}], more formally is [XML Schema Definition Language] ([XSD]) specifies the [XML] [Schema] Definition Language, which offers facilities for describing the structure and constraining the contents of [XML] documents, including those which exploit the [XML] Namespace facility. 

The schema language, which is itself represented in an [XML] vocabulary and uses namespaces, substantially reconstructs and considerably extends the capabilities found in [XML] document type definitions (DTDs). 

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [XML_schema|Wikipedia:XML_schema|target='_blank'] - based on information obtained 2018-01-08- 
* [#2] - [W3C XML Schema Definition Language (XSD) 1.1 Part 1: Structures|https://www.w3.org/TR/xmlschema11-1/|target='_blank'] - based on information obtained 2018-01-08-