Checks a theme's support for a given feature before loading the functions which implement it.
<?php require_if_theme_supports( $feature, $include ) ?>
Let's assume you would like to support a custom feature 'my_theme_feature' in your theme. That feature is implemented in the file my-feature.php and enabled when the feature was registered by add_theme_support(). The next code will load my-feature.php from theme's directory when a feature 'my_theme_feature' was registered.
<?php require_if_theme_supports('my_theme_feature', get_template_directory() . '/my-feature.php'); ?>
require_if_theme_supports() is located in
Theme Features: sidebar, menus, post-formats, title-tag, custom-background, custom-header, custom-logo, post-thumbnails, automatic-feed-links, html5, editor-style, content_width