UPS-assigned tracking number assigned to the whole shipment .
If the shipment contains only one package (PackageCount = 1), the MasterTrackingNumber is the same as the first PackageTrackingNumber (where PackageIndex = 1). This property is returned in the server response for track requests other than tracking by Reference Number (i.e., IdentifierType is set to a value other than itReferenceNumber).
The MasterTrackingNumber uniquely identifies a shipment up to 18 months. However, it is possible that this number may be reused within a shorter time period. If this occurs, a query by Master Tracking Number returns all shipments (from the same ShipperAccountNumber) matching this number.
This is not available for freight shipments.
This property is read-only.