References (other than tracking number) assigned to the package at PackageIndex by either shipper or customer.
This should be inspected after a call to the TrackShipment method.
For FedEx, when populated, it is a string in this format: "Reference Type: Reference Value" pairs (separated by ';' if more than one reference is found).
For UPS, when populated, it is a csv string. A package can have 0 to many reference numbers assigned to it by the shipper when the shipping label was generated.
Currently, USPS does not support multiple package shipments and this does not apply.
This property is read-only.