Lets say i have a business data modell that has person and also puppy. Each object can exist independent from each other, however person have a puppy object, that can be null, it is not mandatory.
I have a process, and a business variable, the type is person. I give the person some data, and an existing puppy object. That means i will have a new line in the person table, and one existing line from the puppy object will be referenced to this line by persistenceId (if i understand this correctly).