When you ingress initial data to Customer 360 from Salesforce, the ingress job creates master records for all the Salesforce records in Customer 360. The ingress job assigns a business ID to each master record in Customer 360.
After you onboard the records to Customer 360, you can process the master records. When you run a match and merge job, the job matches the records in Customer 360 and merges the duplicate records to update the master records. The unique records of Salesforce retain the business IDs of the master records, and the duplicate records of Salesforce use the business IDs of the updated master record.