title: Wordpress 'twentyseventeen' theme filters declarations
body: <p>I've just started learning Wordpress and going through standard/default themes. If i have understood filters idea correctly, before we can apply them we need to add callback functions via <code>add_filter($hook, $callback, $args)</code>. However looking at the 'twentyseventeen' theme i can't see those declaration for <code>twentyseventeen_starter_content</code> and it is used then with: <code>$starter_content = apply_filters( 'twentyseventeen_starter_content', $starter_content );</code> (file functions.php) and <code>twentyseventeen_front_page_sections</code> - <code>$num_sections = apply_filters( 'twentyseventeen_front_page_sections', 4 );</code> (file front-page.php). What am i missing and how does it work without setting callback functions?</p>