An XML structural construct. A mechanism for embedding information in a file intended for proprietary applications rather than the XML parser or browser. The XML parser passes the instructions to the application. A processing instruction is a string of text included almost anywhere in an XML document's character data between ? and ? marks. It begins with the name of the application for which the PI is intended, followed by the data for the instruction. An example is the XML declaration that begins every valid XML file: ?xml version="1.0" standalone="yes" ?