This article explains what a callback URL is and how to use it with the Vizion API.
The callback URL is used as a 'return to sender' type address. The end user provides the required input and when an update is available for your shipment Vizion will send it to you.
To begin the process of receiving updates to your callback URL you will submit your input including the [callback_url] parameter. The value should contain the URL at which you wish to receive updates for your shipment.
Once this relationship is established Vizion will continue to send updates to your callback URL until the reference has been unsubscribed from.