What is the purpose of XML <??> tokens?

Typically, an XML document starts with a title, for example <?xml version="1.0" encoding="utf-8"?>

Consider this document

   <?xml version="1.0" encoding="utf-8"?>
    <Foo>
        <?One?>
        <?Two some?>
        <?Three <?Nested ?> ?>
    </Foo>

• Is it possible to reuse <?and reuse ?>inside an XML document?
• Does the XML parser ignore this content in these tags? How to get them?
• Is there a problem with the Three Nested tag?

I assume this is a document property and can be placed anywhere. Correctly? I am also going to create some XML data based on these meta tags. Is this the right approach? eg.

  <?if a == b ?>
  <Node>IncludeIfEqual</Node>
  <?endif ?>

PS Google does not help. It ignores the keyword <?.