WordPress at a glance
filter is not described

woocommerce_email_subject_customer_invoice_paid filter-hook . WC 1.0

Usage

add_filter( 'woocommerce_email_subject_customer_invoice_paid', 'filter_function_name_8372', 10, 3 );
function filter_function_name_8372( $format_string, $object, $that ){
	// filter...

	return $format_string;
}
$format_string
-
$object
-
$that
-

Where the hook is called

WC_Email_Customer_Invoice::get_subject()
woocommerce_email_subject_customer_invoice_paid
woocommerce/includes/emails/class-wc-email-customer-invoice.php 86
return apply_filters( 'woocommerce_email_subject_customer_invoice_paid', $this->format_string( $subject ), $this->object, $this );

Where in WP core the hook is used WooCommerce

Usage not found.