XML Process Instructions
XML process instructions are normally included in the XML document prolog, commonly thought of as the header in HTML. Processing instructions may be placed anywhere in the document so long as they are outside the element tags.
The rules for the names of process instructions are similar to the rules for element names, however in the case of process instructions there are some reserved xml process instructions such as "xml-stylesheet".
The process instruction syntax:
<? target instruction ?>
The target is the application name the instruction is meant for. It can be a reserved value such as "xml-stylesheet" or the name of an external application such as a script program name.