Hey @karstenkoehler!
Hopefully we will alreday have all the Custom Object Records in a couple might be missing though.
The logic that determines the association is if the Referrer Code on Contact Record matches a Partner Code (Object ID Record) on the Custom object then associate them together. So they will be associated with both records i.e. the custom object record will have multiple contacts and the contacts should only have 1 custom object record.