In the world of information technology, a system identifier is a specific kind of “document processing construct” – it tells computers how a specific file should be interpreted by identifying what kind of application should use it. These identifiers go in the beginning of a file to give directions to the interpreting computer. Programmers use this kind of helpful directive in markup languages like HTML and XML.
In the beginning, system identifiers were used in a HyTime markup language environment for tools like Standardized General Markup Language or SGML. The identifier is also part of an HTML document, the common type of file for creating web page displays. In addition, it is part of XML, a markup language for controlling virtual elements with tags.
System identifiers are generally included in a Document Type Declaration(DTD). The DTD is part of an orienting label that goes before the executable code. Programmers call it a fragmentless URI reference. The system identifier may be part of what is placed within quotation marks in a DTD.
Different kinds of identifiers help computers use markup files in different ways. A system identifier is for a document that should only be used by one application. When a document spans more than one application, it uses a public identifier.
System identifiers will often include a reference to the World Wide Web Consortium, or W3, through a tag including www.w3.org. The World Wide Web Consortium is a group presenting international standards for markup languages to the programming community. The W3 is an authoritative source for learning the structure of XML and HTML documents. That’s why a system identifier will include the direction to the W3 address.
The identifier and other parts of the Document Type Declaration, or DOCTYPE, are interpreted by Web browsers. Programmers refer to the diagnostic actions of Web browser technologies as “sniffing” or “switching,” where the browser determines a layout mode with attention to the DTD code. In many kinds of modern web browsers, the DTD is largely useless if the HTML type is interpreted with HTML parsers, instead of other reading methods. Nevertheless, the system identifier and DTD are still the common standard for making sure the web browser recognizes the format of a file. All of this creates the sophisticated system where a variety of browsers accurately lay out web pages according to the source code that is presented to them.