The Merge By field applies only to the Import/Merge function and is used to determine if records in the Import file also exist in the Keystroke database.
If the value of the Merge By field in an Import File record matches the value of the same field in a Keystroke database record, the record is import/merged.
If the Merge By field is set to None, the Import/Merge function will not operate correctly.
See:
Data Importer Contents