Here you will find information on the relationship between a parent and child reference ID.
A parent_reference_id is present when a container reference is generated from a master bill of lading reference. When tracking of containers is initiated from a master bill of lading number all references created at the container level will now show the master bill of lading reference_id as the containers parent_reference_id.
The parent_reference_id field is only found in reference updates and is only populated if a container reference is generated by a master bill of lading reference. This will allow you to easily group shipments together as you receive updates for each container.
NOTE: If a reference is created with a container ID the parent_reference_id value will be null.