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. 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.
To retrieve this property corresponding to each package, the PackageIndex should be set from 0 to PackageCount - 1.
This property is read-only.