XML what does that question mark mean

Question

I know XML documents usually start with something like:

<?xml version="1.0" encoding="UTF-8"?>

My question is regarding the <? and ?> what do they mean on their own? As in what does:

<?Any old text?>

mean in XML

Thanks

Answer 1

It's a prolog – see 'The XML Prolog' in XML Syntax Rules at w3schools.com.

For more thorough definition see '2.8 Prolog and Document Type Declaration' in 'Extensible Markup Language (XML)' at W3C.