I would like to understand the logic of how the master record is set during the automatic merge process.
Automatic Duplicate Management determines the master record by the following criteria. It goes down the list for importance and once one is true it will set the Master record:
Member ID -is not blank.
Login Date- most recent.
Last modified date- most recent.
Cons ID - is a higher number.
Ex: One record has a member id, but the last login date was 1/1/15 and the other is 8/13/14 (without member id). The master record in this scenario would be the record with the member id even though the last login date is older.