This is based upon the allowable characters in eXtensible Markup Language (XML) tag names, but restricted further to only uppercase characters.
XML stands for Extensible Markup Language.
At its core, RSS is written in Extensible Markup Language (XML) and the first element required in an RSS feed is the declaration: <?xml version="1.0"?>.
With the advent of dynamically generated sites, a great deal of HTML has been replaced by more suitable languages such as eXtensible Markup Language (XML) and Cascading Style Sheets (CSS).
You get the power of MySQL behind the eXtensible Markup Language (XML) of your site, with a few simple keystrokes.