A processing instruction is used to directly pass on some information or instruction to the application via the parser.
<?my-application some instructions ?>
The token after the initial question mark (here
my-application) is called the target and identifies the application at which the instruction is aimed. What follows it is not further specified and it is up to the application to interpret it. Entity and character references are not recognized.
It can appear at the top-level, or in element content.