Item 17: Use processing instructions for process-specific content

• Intended only for a very particular process, often a purely local one

• Describes how a particular process acts on the data in the document

• Does not describe or add to the content itself

• Instruction is a unit that can be treated in isolation; does not extend across a part of the XML document.

• Data does not need to be validated.

• Content is not XML-like.

• Applies to the entire document