Hi,
$this->payment_params->verified_status comes from your own payment plugin.
Normally, you have such attribute at the beginning of your payment plugin class:
var $pluginConfig = array(
'pbx_site' => array('Site', 'input'),
'pbx_rang' => array('Rang', 'input'),
'pbx_indentifiant' => array('Indentifiant', 'input'),
'cancel_url' => array('CANCEL_URL', 'input'),
'return_url' => array('RETURN_URL', 'input'),
'invalid_status' => array('INVALID_STATUS', 'orderstatus'),
'pending_status' => array('PENDING_STATUS', 'orderstatus'),
'verified_status' => array('VERIFIED_STATUS', 'orderstatus')
);
where you define the options of the configuration of the payment method and there you can see the line:
'verified_status' => array('VERIFIED_STATUS', 'orderstatus')
which add the dropdown to select the status the merchant wants when a successful payment notification comes in.
The values of the options are then loaded by yourself in $this->payment_params at the beginning of the onPaymentNotification by calling the function $this->loadPaymentParams($dbOrder); as in the example payment plugin.