获取woocommerce 3中自定义结帐字段的值

Question

In Woocommerce checkout, I am adding a custom checkout field and here is my code:

add_action( 'woocommerce_before_order_notes', 'shipping_add_select_checkout_field' );
function shipping_add_select_checkout_field( WC_Checkout $checkout ) {
    $options = array_merge( [ '' => __( 'Nothing to select' ), ], city_zone() );
    woocommerce_form_field( 'billing_country_zone', array(
            'type'     => 'select',
            'class'    => array( 'form-row-wide', 'address-field', 'update_totals_on_change' ),
            'label'    => __( 'City zone' ),
            'required' => true,
            'options'  => $options
    ), WC()->customer->billing_country_zone );
}

Now I am totally lost as I need to know what is WC()->customer->billing_country_zone for and how can I check it's value…

Any help is really appreciated.