What are allowedposttags and allowedtags?

I have been going crazy trying to google up what these globals are. What are allowedposttags and allowedtags? What is the difference between the two? Is there a list of all WP Globals and an explanation of what they are?

For MCE Editor – to allow users not logged in to use lists and underline + strikethrough add following:

add_action('init', 'my_html_tags_code', 10);
function my_html_tags_code() {
  global $allowedposttags, $allowedtags;
    //$allowedposttags["ol"] = array();
    //$allowedposttags["ul"] = array();
    $allowedtags["ol"] = array();
    $allowedtags["ul"] = array();
    $allowedtags["li"] = array();
    $allowedtags["span"] = array( "style" => array() );
}