A parent reference is generated only when a container reference is created from a master bill of lading reference. When tracking of containers is initiated from a master bill of lading all references created at the container level will have a parent_reference_id property set to the parent reference's reference_id. This will allow you to easily group shipments together as you receive updates for each container.
If you are creating a reference via carrier code or ACI, the parent_reference_id value will be null.