Order saving

Saving is always performed using the order's method save(). Not only the order is saved, but also all entities associated with it (payment and shipment) specifically in the same consistent state that resulted after calculation of objects:

/** \Bitrix\Sale\Order $order */
$order = \Bitrix\Sale\Order::load(18);
$order->setField("USER_DESCRIPTION", "Deliver to apartment");

$shipmentCollection = $order->getShipmentCollection();
/** \Bitrix\Sale\Shipment $shipment */
foreach ($shipmentCollection as $shipment)
{
    if (!$shipment->isSystem())
        $shipment->allowDelivery();
}

$order->save();

There are two saving events that can interrupt the order saving process.

Example

//example of event OnSaleOrderSaved use

use Bitrix\Main; 
Main\EventManager::getInstance()->addEventHandler(
    'sale',
    'OnSaleOrderSaved',
    'myFunction'
);

//handler receives the sum to be used for specific actions in the future:

function myFunction(Main\Event $event)
{
    /** @var Order $order */
    $order = $event->getParameter("ENTITY");
    $oldValues = $event->getParameter("VALUES");
    $isNew = $event->getParameter("IS_NEW");

    if ($isNew)
    {
        $sum = $order->getPrice();
        // . . . 
    }
}