This global variable is used to store elements that are allowed in a post. These elements are allowed only in body of the post, allowedposttags is used for rest of the HTML. All others are stripped before the post is published. It is an associative array set in [wordpress root folder]/wp-includes/kses.php (kses is a HTML/XHTML filter written in PHP). It is used in function wp_filter_post_kses($data) defined in [wordpress root folder]/wp-includes/kses.php which is eventually executed for the action
Refer to allowedposttags on ways of modifying the elements that are allowed in the post.
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