Adding A New Product Data Tab

functions.php

<?php
/* Register New Tab */
add_filter( 'woocommerce_product_data_tabs', 'register_internal_comments_data_tab' );
function register_internal_comments_data_tab( $data_tabs ) {
    $internal_comments = array(
        'internal_comments' => array(
            'label' => __('Internal Comments', 'spyr'),
            'target' => 'internal_comments_data',
            'class' => array(),
        ),
    );

    // Return default tabs plus our new one
    return array_merge( $data_tabs, $internal_comments );
}