Explaining comment_reply_link nuance

gistfile1.php

<?php
/**
 * Very simplified comment callback to illustrate one particular nuance. Yours should be more robust.
 * 
 * param $comment object  The comments object
 * param $args    array   Array of parameters passed to wp_list_comments
 * param $depth   integer How deep we are in the nested comments
 */
function tdd_demo_comments_callback( $comment, $args, $depth ) {
?>
<li <?php comment_class() ?> id="comment-<?php comment_ID(); ?>">
	<div id="div-comment-<?php comment_ID(); ?>" class="comment-body cf">
		<?php // Do whatever output of the comment info ?>
		<?php comment_reply_link( array_merge( $args, array( 'add_below' => 'comment-footer' ) ); ?>
	</div>
	<div>
		<?php // For the example, say we're outputting other stuff, perhaps an avatar here ?>
	</div>
	<div id="comment-footer-<?php comment_ID(); ?>" class="comment-footer">
		<?php // maybe something else here ?>
	</div>
	<?php // When it's moved by the javascript, the comment form will be inserted here ?>
</li>
<?php } ?>