A processing instruction (PI) is an SGML and XML node type, which may occur anywhere in a document, intended to carry instructions to the application.
[1][2] Processing instructions are exposed in the Document Object Model as Node.PROCESSING_INSTRUCTION_NODE, and they can be used in XPath and XQuery with the 'processing-instruction()' command.
[6] The most common use of a processing instruction is to request the XML document be rendered using a stylesheet using the 'xml-stylesheet' target, which was standardized in 1999.
The DocBook XSLT stylesheets understand a number of processing instructions to override the default behaviour.
[8] A draft specification for Robots exclusion standard rules inside XML documents uses processing instructions.