Output the Tab's content

functions.php

<?php
/* Output the Tab's content */
add_action( 'woocommerce_product_data_panels', 'internal_comments_data_panel' );
function internal_comments_data_panel() {
?>
    <div id="internal_comments_data" class="panel woocommerce_options_panel" style="display: none;">
        <div class="options_group">
            <?php
            woocommerce_wp_textarea_input(
                array(  'id' => '_internal_notes',
                        'label' => __( 'Internal Notes', 'spyr' ),
                        'desc_tip' => 'true',
                        'description' => __( 'Internal note about this product that is never displayed on the Front-end.', 'spyr' )
                )
            );
            ?>
        </div>
    </div><!-- #internal_comments_data_tab -->
<?php
}