This filter is applied to use the payment subtotal amount data of orders, it is including two parameters.
$this->subtotal is got the subtotal value.
$this->ID is used to get id.
An example of how you can append additional information:
function get_subtotal( $subtotal ){ // return subtotal value; return $subtotal; } add_filter( 'rpress_get_payment_subtotal', 'get_subtotal',10, 2 );