Each object in a bundle has a unique reference. This reference is defined in the attribute reference. A reference is comprised of the child IDs of the parent objects and of the object itself.
Example
790B817A-B2C3-475E-86E5-08118150EA94:308DFBC5-F520-414E-ACBD-0A4495C33301_EAAC39D9-81BA-476B-A003-039ABF3618AA